While the exact specifications of this gene are never truly explained, it is generally assumed that the gene allows for a person to have a parasite/host type relationship with a malignant spirit referred to as a "devil" as well as having it (the gene) control the transformations of one to the other. The exact requirements for these transformations are never fully revealed. The spirits can possess the person at any given time during the person's lifespan, even while still in their mother's womb or (apparently) just after death.
The people currently carrying the Devil Gene are, in the order in which it was "revealed": Kazuya Mishima, Jin Kazama, Azazel, and 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 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 Jinpachi didn't possess the Devil Gene and that a revengeful ghost possessed him. Interesting though, in both console versions of Tekken 5, Jinpachi's ending states that he's possessed by the Devil. Also, in Jinpachi's interlude with Heihachi, the latter theorizes that his father has the Devil's blood and finally, in the character history section of the Tekken 6 CG art book, it states that Jinpachi has the Devil Gene. It says that he is an unique link within the Mishima bloodline, in which the Devil Gene appears after him. When fans mention this on Harada's Twitter page, he reveals that Tekken project team dares to mislead fans (sometimes). He also said that the Tekken stories progress in parallel with each character, and that it only has one true canon (story setting).
The Devil "Possession" Timeline
Kazuya had his encounter with the devil, later to be known as Devil by name, when he was about 5 years old. Heihachi, sickened by his weak and emotionally soft son, threw him off a cliff in order to have him climb back up. Although the injuries nearly killed Kazuya, he sold his soul to Devil and was able to climb back up the cliff. This is the first time the Devil Gene was presented in the Tekken series.
After the events of Tekken 2, Heihachi threw Kazuya's body into a volcano. This inadvertently caused Devil's soul to be split into two parts; one that stayed with Kazuya and the other that went in search of Kazuya's yet unborn son, Jin Kazama. When it finally found the unborn Jin, it was warded off by his mother, Jun Kazama, and was unsuccessful in taking over Jin. However, 15 years later, it returned, branded Jin's left arm with a mark and (possibly) brainwashed him (as seen in the Tekken 3 opening).
The Common Denominator
Interestingly enough, while Heihachi does not seem to have the powers of the Devil Gene, he has been the catalyst whose actions have resulted in the devil possession of the other Mishimas.
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 demonic possession is unrelated to the Devil Gene). It was also he, who threw Kazuya off the cliff when Kazuya was a child, almost killing him, causing the Devil gene to surface within Kazuya. Several years later, after the events of the second tournament, Heihachi threw Kazuya into a volcano, thus releasing part of Devil, which then searched for and tried to possess Jin Kazama, though initially unsuccessful. Heihachi and his Tekken Force soldiers fatally shot Jin Kazama near his death after the conclusion of Tekken 3, causing the Devil Gene to awaken within him and partially transform Jin into his Devil self.
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 side of the person becomes more dominant, to the point of changing the physical appearance of their host, who in turn becomes much stronger.
- 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. Jin Kazama seems to have become able to activate his Devil Gene consciously by the time of the events in Tekken 6 without becoming a devil like that of his father.
- 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 hatred he felt for Heihachi's betrayal and evil.
- Activation by external stimuli: Jin partially transforms at the end of the events of the fourth tournament due to Devil's attempts at forcing him to (as seen in Jin's Tekken 4 ending). Perhaps no more than half an hour later, when Jinpachi and his dark energy is freed from under Hon-Maru, Jin completely transforms because of his reaction to Jinpachi's evil energy, meaning that the Devil Gene can come out whenever and wherever there is negative energy around.
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 explained, often not at all. So, it remains unknown how they suppress the Devil Gene; it is only known that they are able to do so.
Before the concept of a Devil Gene was created, and the concept was instead strictly spiritual, Jun Kazama seemed able to suppress the Devil inside Kazuya. It was explained as having been because of Jun's serene and pure being. However, when the Devil history was reconnected and the Devil Gene concept created, the influence of the Kazamas became genetic as well; their bloodline, rather than Jun specifically having been a special case.
At the conclusion of Tekken 4 the effect of the bloodline is shown as Devil tries to transform Jin into his devil form by using the anger he has for his Mishima family, but fails and theorizes that it is due to the Kazama blood running in Jin's veins.
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.
Before the concept of the Devil Gene, Angel was able to suppress the devil side of him. However, since the creation of the Devil Gene concept, 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 discovered strange malignant cells in it that they referred to as the "devil cells". With this knowledge, they used Kazuya to further their research and commenced a project known as the (Devil-Human) Integration Program. Using Kazuya's DNA they attempted to combine the devil and human cells, producing a half human half devil organism using human test subjects. But when trying to combine the two, the devil cells destroyed the human cells, making it impossible to create a "devil human", a viable life form similar to Kazuya's genetic make up.
G Corporation 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 similar or can be referred to as G Corporation's 'Anti Devil Factor'. It is also revealed in the comic that Kazuya and Jin use the 'Anti Devil Factor' to suppress the influence of the 'Devil' (however, 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).
While the powers of each of the "Devils" are different, they 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 exactly the same as Kazuya Mishima's fighting style, since Devil was merely a P3 costume for Kazuya in Tekken 1. In Tekken 2, Devil gained wings and with them, the power of flight (though this is sparingly 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 separate Devil and Kazuya in terms of gameplay, along with the back throw Atomic Drop. In Tekken Tag Tournament, Devil had several moves 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, until he becomes one with Kazuya as his transformation mode as of the end part of Tekken 4, starting from this form's return in Tekken Tag Tournament 2. Devil can also use telekinesis (this is not one of his moves, and it was only seen in Kazuya's Tekken 4 epilogue), despite losing some of his power to his son, he is still very powerful.
Devil Jin, like Devil, can also fly and fire lasers from the third eye on his forehead, however, their fighting styles 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 Dark Resurrection win animations, where Devil Jin holds the opponent up in the air.
Devil Jin has shown to have amazing feats, he has the power to destroy an entire forest just by force alone. He also has the power to take powers just like that of other Devil Gene users to make it their own.
Oddly, Jin feels pain whenever he transforms, despite the fact that the Devil Gene has healing attributes.
Azazel appears to have the abilities to summon boulders (in original Tekken 6 arcade before it was removed) and/or spikes (in Bloodline Rebellion and the console version) in 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. He also possesses a tail sweep attack resembling Roger Jr.'s; however, he whips it twice in a row. His power source is an orb, which comes from inside his body. This orb has the ability to turn any fighter who touches it into another host of the devil and it may kill any fighter who destroys it while inside Azazel's body instantly as seen in the endings of Hwoarang, Baek, Wang, Yoshimitsu, Ganryu, Dragunov and Raven.
- Kazumi Mishima is the first and only known confirmed female who carries the Devil Gene.
- According to Tekken producer Katsuhiro Harada, Kazuya was not the first Mishima family member to make contact with the devil, and that both Kazuya and Jin believe it started with Jinpachi but only Heihachi really knows who it was. Harada also pointed out that neither Jinpachi, Heihachi nor Lars possess the Devil Gene. This hints towards Kazumi Mishima being the first to have started the Devil Gene bloodline.
- It would also explain the reason why Jinpachi, Heihachi and Lars do not possess it yet both Kazuya and Jin do.
- Jin Kazama and Kazuya Mishima are by far the only fighters in the Tekken series to control the Devil's power, though Kazuya, after he and Devil became one, seems to have become more and more evil from then on.
- In Street Fighter X Tekken, if Ryu defeats Kazuya, his winquote has him wonder if the Devil Gene and Satsui no Hado are related. It should be noted, however, that the Satsui no Hado is more energy-based and greatly changes the warrior into a more violent demonic fighter at the potential cost of his humanity.
- It is revealed that Steve Fox may also possess the Devil Gene, though there is no physical evidence, other than a scar that might or might not be related. In his Tekken Tag Tournament 2 ending, it shows that the Mishima Zaibatsu had tested the Devil Gene power against him after he was created but Steve's story is still yet to be revealed. Even though Tekken Tag Tournament 2 is a non-canonical game, Steve's ending could be canon to his main storyline.
- This is easier to notice in Leo Kliesen's ending cinematic, as when she downloads the Mishima Zaibatsu's database onto an USB device, she stumbles upon a file containing information on Devil Gene research, which has been cancelled long ago due to the subjects dying. Unknown to her, the subject NTO1 (which is still alive) is Steve.