The Devil Gene appears in Capcom's Devil May Cry and Namco's Tekken. While there are differences in the way it appears in different gaming franchises the one common trait of the Devil Gene remains the same. It allows a person to overcome their physical limitations by transforming themselves into a more demonic form. This grants enhanced strength and speed to its user and extra abilities such as flight and the firing of lazer beams.
In Namco's Tekken franchise, the Devil Gene has unknown origins. The Tekken series producer, Katsuhiro Harada, has stated that although its first appearance was in the first Tekken game, courtesy of the being referred to as Devil, that its origin has yet to appear in a Tekken game. The Tekken 6 final boss, Azazel, also has an unknown relation to the Devil Gene.
Two characters in the Tekken series have the Devil Gene, Kazuya Mishima and Jin Kazama. Harada also stated that while Jinpachi Mishima has a demonic transformation in Tekken 5, it is unrelated to the Devil Gene.
Kazuya gained the Devil Gene as part of a pact between himself and "Devil," exchanging his soul for the power to defeat Heihachi Mishima. Devil would pass on the Devil Gene and some of his power to Kazuya's son, Jin. While Kazuya and Devil work as a partnership, using the Devil Gene when necessary, Jin has no control over his devil powers.
This is made apparent in Tekken TAG Tournament 2, in which Kazuya can transform into his devil form at will and unlike previous games, isn't a separate character. This is due to events in Tekken 4, in which Kazuya tricked Devil into a "unifying" of their powers. Kazuya and Devil are now one being and Kazuya's soul is his own again. On the other hand, Jin and Devil Jin are always separate characters. There is no entity within Jin to restrain and control his devil form, which forces Jin to transform in moments of great stress.
Devil May Cry series
There are four ways in which to have the Devil Gene in Capcom's Devil May Cry games. The first is to be a relation of a demon, such as the sons of Sparda, Dante and Vergil. The second is to be created as such, as the ruler of the Demon World, Mundus, created Trish. The third is through experimentation, either by manipulating your own DNA into resembling a demon, like Arkham, or by experimenting on demonic weapons and the souls of demons, like Agnus. The fourth is to have a Demon's soul bind itself to you, which in Nero's case, is the aforementioned Vergil on his arm.
In the Devil May Cry universe, the Devil Gene transformation is known as Devil Trigger, activated by "Devils Arms" such as the Rebellion, Yamato and Alastor. Like the super saiyan transformations in Dragon Ball Z, they are initially "triggered" by high emotional stress but are natural later once the form has been adapted to. Nero's Devil Trigger form changes him little, gaining an aura and red eyes, much like Akuma and Evil Ryu from another Capcom franchise, Street Fighter. His power increase mostly comes from an ethereal warrior fighting a long side him who resembles Vergil as "Nelo Angelo."
Dante and Vergil both change into a partial demon form, red and blue respectively. It has been shown that other than the greatest demons of the Demon World the only people who can fight evenly with Devil Trigger Dante and Vergil are themselves. This can leave them temporarly weakened however, enabling lesser beings like Arkham to successfully sneak attack them.
As the sons of Sparda, Dante and Vergil have access to a full Sparda Devil Trigger form through the Sparda Sword. Vergil tried to do this in Devil May Cry 3 but was defeated by Dante and later corrupted by Mundus into a controllable Demon form, Nelo Angelo. Later in the events of the first Devil May Cry, Dante would kill Nelo Angelo, transformed the Force Edge in the Sparda Sword and us the Sparda Devil Trigger to defeat Mundus.