The Devil Gene (デビルの血 Debiru no Chi, lit. Devil's Blood, "Devil Cell", Devil Factor or Devil's Consanguinity) - although referred to as the Devil's Power (デビルの力 Debiru no Chikara) in the Tekken 6 Scenario Campaign - Was first introduced in Tekken 4.
The Devil Gene's exact meaning and specifications were not fully explained in Tekken 4, 5 and 6. According to the CGI movie Tekken: Blood Vengeance, it is explained that the Devil Gene is a cataclysmic Gene that strengthens its cellular compounds to ten times to that of a normal human. Within Kazumi Mishima Kazuya Mishima and Jin Kazama, it increases their physical strength. According to Tekken Tag Tournament 2, the Devil Gene is a hybrid gene that instantaneously activates the body's own cells, granting the subject an immeasurable boost in power, superhuman strength.
The people that currently possess the Devil Gene are, in the order in which it was "revealed", Kazuya Mishima, Jin Kazama, Azazel, and, most recently, Kazumi Mishima. Heihachi Mishima doesn't seem to possess the Devil Gene like the other members of his family. It was previously believed for a long time that Jinpachi Mishima also possessed the Devil Gene due to his appearance dialogues and his ending in Tekken 5, but it was revealed by Tekken director, Katsuhiro Harada's, twitter page that the Tekken project team dared to mislead fans. He also said that the Tekken story progresses in parallel, and that Tekken only has one canon (story setting) and if Tekken only has one canon story then it will be boring. This means that at some point the Tekken team project working in the development of Jinpachi's Tekken 5 story could have meant for him to have the Devil Gene but his story was later changed in a parallel continuity. After pointing out that Jinpachi, Heihachi, and Lars did not possess the Devil Gene, fans also asked Harada (via twitter) about the origins of the Devil Gene, he stated that Kazuya was not the first Mishima to have the Devil Gene and that Kazuya and Jin thought it started with Jinpachi. He points out that only Heihachi knows the truth, only Heihachi knows who the first person is in the Mishima family to make contact with the Devil. This lead many fans to theorize that it started with Kazumi Mishima, the wife of Heihachi and the mother of Kazuya. Theoretically if so, this would explain why Kazuya and Jin only possess this strange malignant Gene in the Mishima family. This all turned out to true and confirmed as Tekken 7 would reveal that Kazumi was the first person in the Mishima family to have the devil gene and Kazuya would inherited the devil gene at birth from his mother and passing it on to Jin as well. In Tekken: Blood Vengeance it is stated that the Mishima Zaibatsu experimented on Shin Kamiya using Kazuya Mishima's blood for unknown reason, though it is possible they could have wanted to integrate the Devil gene with M-Cell. In Tekken Tag Tournament 2 Leo Kliesen's ending it is revealed that Steve Fox, dubbed NT01, was created by the Mishima Zaibatsu for the sole purpose of the Devil Gene Research project. Steve Fox who was in the care of Emma Kliesen was said to have died at the age of 6 during experiments. However, Steve is well and alive, Emma Kliesen could have misleaded the Mishima Zaibatsu concerning Steve's whereabouts. It is unclear if this is canon to current Tekken story because Tekken Tag Tournament 2 is non-canonical to the main series of Tekken. However, in Tekken 5 it was revealed that Steve was an actual creation by the Mishima Zaibatsu, but the purpose behind Steve being created has yet to be revealed in the canon story.
Tekken 7 introduced Kazumi Mishima to the table, and trailers and her transformation in-game is proof that she does have the Devil Gene, but her appearance and powers act differently in comparison to Jin and Kazuya.
The Devil "Possession" Timeline
Kazuya was the first Mishima shown to possess this malignant power in the series. But Katsuhiro Harada stated that Kazuya was not the first Mishima to possess this power or make contact with the Devil Gene. He goes on to say that there was another person within the Mishima family to possess the Devil Gene before Kazuya, and Heihachi is the only "one" who knows this secret and truth. Before the events of the first tournament in Tekken 1 it is incorrectly speculated that at the age of 5 Kazuya sold his soul to the Devil when Heihachi Mishima, threw him down a cliff. However according to Kazuya's prologue in the Tekken 1 manual booklet it never stated nor mention Devil. But in the non-canonical Tekken: The Motion Picture at the age of 12 Kazuya was thrown down a mountain and was approached by Devil and was offer strength for the cost of his soul to exact his revenge against his father. Many fans adopted the "Tekken motion picture" concept of Kazuya selling his soul to the Devil in the games series explanation to Kazuya link with having the Devil Gene like in the first game, however, it should be noted that this particular movie is non-canonical to the main series, and the origin behind Kazuya carrying the Devil Gene remains to be explained in the main series, however, in regards to the origin of the Devil Gene within Kazuya Mishima, Katsuhiro Harada, creator of the Tekken series stated via twitter that he will reveal how Kazuya got the Devil Gene in the Tekken 20th Anniversary 
With the Tekken 7 announcement, it is revealed that Kazuya was born with the Devil gene. After the events of Tekken 2 Kazuya was defeated by Heihachi and thrown into a mouth of a volcano. Sometime later Jun Kazama found herself pregnant with Kazuya's son before he left her, Jin, and Devil appeared before Jun herself, threatening to possess the body of her unborn child, however, Jun defeated Devil, and Devil failed to possess Jin. A scene in the opening of Tekken 3 shows a ghostly figure that appears to look similar to Kazuya's Devil form is shown branding a strange tattoo on Jin's arm, many fans theorize that Devil possess Jin in that scene, however it is unconfirmed. Notably, Unknown, and, later, Jun Kazama, also have the same tattoo as Jin, but Kazumi and Kazuya do not have the mark, hinting that this issue may be Kazama-related instead. Because it is revealed in Tekken 7 that Kazuya was born with the Devil gene, this could mean that Jin inherited the devil gene as well through birth. In Tekken 6 within the scenario mode if you play as Kazuya, Jin, and Devil Jin on Azazel stage, Azazel states that he predicted that Kazuya and Jin would bring chaos to the world to awaken him and bring the world to an end. Also on the stage of Azazel while playing with Devil Jin, he states that his power and Azazel's power comes from the same source, and that he is no longer Azazel puppet. This could mean that Azazel may be responsible for the origin of the Devil Gene in the Mishima family. Katsuhiro Harada stated on his twitter page that Azazel is one of the roots of Devil and the important thing to keep in mind is who was the first Mishima to contact Devil. It is unclear to what Harada meant by saying this. It was later revealed with the Tekken 7 teaser that Kazuya's mother is, in fact, the first person in the Mishima family to possess the Devil Gene.
The Common Denominator
Interestingly enough, while Heihachi does not seem to carry the Devil Gene, he has been the catalyst whose actions have resulted in the Devil within the Mishima family.
Heihachi staged a coup d'etat against Jinpachi, and locked him away under Hon-Maru, which resulted in Jinpachi being possessed by the "monster" (though this possession is unrelated to the Devil Gene). It was also he, who threw Kazuya over the cliff when Kazuya was a child, (possibly) causing the Devil Gene to activate within him. Several years later Devil appears before Jun and her unborn son Jin and tries possess him but failed. Heihachi and his Tekken Force soldiers fatally shot Jin Kazama near his death after the conclusion of Tekken 3, causing the Devil Gene to activate within him, healing him, and partially transform Jin into his Devil form as he defeated them all.
Activation of the Devil Gene
It is not explained how the Devil Gene is activated, but evidence within the games seem to suggest that it can be activated consciously, subconsciously, or based on external stimuli. Regardless of which, the effects seem rather clear; the Devil form of the person becomes more dominant, to the point of changing the physical appearence of their host, who in turn become more powerful.
- Conscious activation: Kazuya performs a conscious activation of his Devil Gene between the events of Tekken 4 and Tekken 5, during the attack of Hon-Maru by the Jack robots, and also tranforms during the Blood Vengeance film. Jin Kazama seems to have become able to activate his Devil Transformation consciously by the time of the events in Tekken 6. Kazumi Mishima, if defeated in a round as her regular self in Tekken 7, will transform into her devil form, and will stay that way for the remainder of the fight.
- Subconscious activation: Jin's first transformation into Devil Jin at the end of the events of the third tournament was arguably a subconscious activation of his Devil Gene, likely in part of the anger he felt for Heihachi's betrayal and the fear of death. Jin also transformed again in Tekken 5 likely because of anger over the loss to Hwoarang. When Kazuya was thrown down the mountain by Heihachi, he was in a near death exprience, anger by his father's bad teachings, the Devil gene begin to activate within Kazuya, thus gaining ultimate power to exact vengeance, and eventually seeking world domination.
- Activation by external stimuli: Jin partially transforms at the end of the events of the fourth tournament due to Devil trying to force him to transform, as seen in Jin's Tekken 4 ending. Perhaps no more than half an hour later, when Jinpachi is freed from under the Hon-Maru does Jin completely transform because of his reaction to Jinpachi's dark energy.
Suppression of the Devil Gene
Aside from the suppression of the Devil Gene through sheer force of will, as is implied by the method of conscious, and to some extent subconscious, activation, there are also several external factors that can suppress the Devil Gene as well. The 'exact' nature of these things are however not well explain, often not at all, so it is unknown exactly -how- they supress the Devil Gene, it is only known that they -are- able to do so. The only ones who have shown this
In the Tekken series, the Kazamas has been shown to supress the Devil transformation. This phenomena is first shown with Jun Kazama, she seemed to be able to suppress the Devil Gene inside Kazuya. It was explained as having been because of Jun's serene and pure being. Later in the series, the explanation of this phenomena seem to be a genetical trait in the Kazama bloodline, rather than Jun specifically having been a special case.
At the conclusion of Tekken 4 the effect of the bloodline is shown as the Devil Gene tries to transform Jin into his Devil form by using his anger he has for his father and grandfather, and theorizes that it is due to the Kazama blood running in Jin's veins.
Asuka Kazama, in her Tekken 5 ending, was also able to suppress the Devil Gene of Jin, however, the ending was non-canon and Asuka never encountered Jin during the fifth tournament. It is unknown how strong an effect she has on the Devil Gene. In Tekken 6's Scenario Campaign, it should be noted that Devil Jin felt repelled by Asuka Kazama's presence, even before meeting her and expressed an extreme disturbance when confronted by her.
While it isn't known how, the Mishima Zaibatsu were successfully able to create chains of an unknown material that was able to specifically suppress the Devil Gene.
Angel, was able to suppress the devil form of Kazuya. However since the debut of Tekken 4 Angel has all but vanished from the Tekken storyline and it is unknown whether or not she is still able to do suppress Devil or if she even still exists.
In Tekken 4 When G corporation discovered Kazuya's body, they discorvered strange malignant cells within Kazuya's body that they refer to as the devil cells. With this knowledge they use kazuya to further they research and creating a project known as the (Devil-Human) Intergration Program. Using Kazuya's DNA of the devil cells they attempt to combine the devil and human, creating a half human half devil organism using human test subjects. but when trying to combine the two, the devil cells destroy the human cells, making it impossible to create a "devil human", a viable life form similiar to kazuya's genectical make up. G coporation used Julia Chang's GENOCELL Program and discovered that the GENOCELL can suppress the devil cell allowing the human cells to combine with the devil cells. In other media such as the Online Tekken Comic the GENOCELL is also smiliar or can be refered to as G Coporation's 'Anti Devil Factor' it is also reveal in the comic that Kazuya and Jin use the 'Anti Devil Factor' to supress the influence of the 'Devil', in Kazuya's case it allows him to also control the power of the 'Devil'/Gene without the conscious influence of it consuming his mind.
Powers / Abilities
While the powers of each of the "Devils" are different, the "Devils" all fight using a form of Mishima Style Fighting Karate except Azazel. This can however simply be attributed to the fact that they use the martial art known by the possessed person.
Devil's fighting style was initially the exact same as Kazuya Mishima's fighting style, since Devil was merely a costume swap for Kazuya. In Tekken 2, Devil gained wings and with them, the power of flight (though this is sparringly used in his moveset). In the same game, Devil also gained a third eye on his forehead, from which he could fire a laser beam. These moves were the first to seperate Devil and Kazuya in terms of gameplay, along with the back throw Atomic Drop. In Tekken Tag Tournament, Devil had several moves that that Kazuya did not have (such as "Devil Fist" (f,F+2), a move where Devil punches his opponent and goes right through them so that he is behind his opponent), and even the moves they did share would sometimes have different properties or ranges, still, the majority of Devil's moves are shared with Kazuya, and he has a very limited moveset, with less than 30 moves in Tekken Tag Tournament, making him the most basic character in the series. Devil can also use telekinesis (this is not one of his moves, and it was only seen in Kazuya's Tekken 4 epilogue).
Devil Jin, like Devil, can also fly and fire lasers from the third eye on his forehead, however, their fighting style are rather different. Devil Jin's fighting style is the mixture of Mishima Fighting Style Karate and Kazama Style Self Defense that Jin used in Tekken 3 and Tekken Tag Tournament. Devil Jin also uses some of the unique moves used by Devil and Angel (such as "Heaven's Door" (f,N,d,d/f+1,u/f), a move that involves performing a strong uppercut, followed by flying high into the air with the opponent, and finally, slamming the opponent's body into the ground). Devil Jin can also use telekinesis, which was first demonstrated in Hwoarang's Tekken 5 epilogue, where Devil Jin uses telekinesis to make Hwoarang's motorcycle explode. This ability is also seen in one of Devil Jin's Tekken 5: Dark Resurrection and Tekken 6: Bloodline Rebellion win animations, where Devil Jin holds the opponent up in the air, choking them. Interestingly, Tekken 6's Scenario Campaign revealed that Devil Jin suffers from massive head pains when Hwoarang or Xiaoyu interact with Devil Jin hinting that gene is still not in full control of Jin.
Azazel appears to have the abilities to summon boulders from the ground to attack the opponent, as well as to keep them away from it, and summoning numerous scarabs made of ice to attack the opponent. Azazel also has some moves that are similar to other bosses in the Tekken series, such as a stun (similar to Jinpachi Mishima's stun attack) and the ability to fire lasers (similar to Devil, Angel, and Devil Jin's laser attacks). Azazel also has the ability to curl up into a ball and roll towards the opponent to attack (like Roger Jr.). He also possess a tail sweep attack resembling True Ogre's, however, he whips it twice in a row.
Kazumi, in her devil form, appears to have her design based on the Kitsune, a mythological white fox with 9 tails, or the Inari, the god of the foxes, whose statues are often seen with the color red as a sign of respect. Her "wings" are made of fire and are based on phoenix wings, her skin becomes pure white, with red tattoos, her horns are colored red, and there are blue crystal-like growths located on her joints. Notably, if she is in rage mode, her crystals will change from their standard blue to red. In her devil form, she is able to fly and hover at startling speeds, is able to conjure fireballs, and can also summon tiger spirits to aid her during a fight, but it should be noted that Kazumi can still summon tiger spirits in her human form. Like preceding characters, she can also fire lasers.