Double Soul

Double Soul, known in Japan as Soul Unison (ソウルユニゾン), is a term used in the Mega Man Battle Network series of video games and MegaMan NT Warrior anime and manga. It is an ability employed by MegaMan.EXE or any NetNavi to gain added powers and techniques in combat using the powers of an allied NetNavi.

Double Soul was first introduced in the two incarnations of Mega Man Battle Network 4. Out of twelve souls, six were exclusive to either game. It appears once more in Mega Man Battle Network 5: Team Colonel and Mega Man Battle Network 5: Team ProtoMan. It replaced MegaMan's Style Change capability of previous iterations.

General
In the closing events of Mega Man Battle Network 3, MegaMan sacrificed himself to save his operator, Lan Hikari from absorption by the horrific Alpha program revived by Lord Wily. Lan thought his ally and brother to be gone forever, only to be revived using data from his grandfather Tadashi Hikari. His revived state, however, rendered him unable to utilize the powerful Style Change programming option any longer. Hence "Double Soul" was developed as a suitable replacement.

Unlike Style Change, which can be performed by a single NetNavi, Double Soul occurs when a Navi's soul communes with MegaMan. This usually happens after battling against the respective Navi in Battle Network 4, or completing a "Liberation Mission" that occurs after having them join the players team (excluding ProtoMan/Colonel, which is obtained near the end of the game instead).

Chaos Unison
Introduced in Mega Man Battle Network 5, Chaos Unison occurs when sacrificing a Dark Chip. MegaMan gains the soul's attributes, and is a different color (usually purple or black). The Charge attack now becomes the same as the sacrificed chip, and doing so does not affect Karma or have the effect of removing 1 HP permanently like using the chip normally would. The catch is that Chaos Unison can only be used for absolutely one turn (cannot be boosted with the Navi Customizer), and when charging, the charge will transition between a Teal-Green to Purple. Firing while purple will release the aforementioned charge shot of the Dark Chip, while firing in green will cause MegaMan to fail to attack. Then, MegaMan will flinch and instantly exit his Unison, becoming normal. Finally, Dark Mega (MegaMan DS) will join the fight on the Virus side of the screen. Dark Mega is invincible and will attack MegaMan for about 13 seconds (a rather unusual nod to an unlucky number) or until the other viruses are defeated, randomly firing at MegaMan and using chips. The Chaos Unison is an immensely powerful transformation, having the attributes of both the power of the Dark Chip and Soul Unison.

Double Soul
In MegaMan NT Warrior Axess, MegaMan is able to perform Double Soul initially through a special bond between fellow Navis; he usually obtained new souls in dire situations, during which either he or the other Navi in question were subject to great peril. After initiating a given Double Soul once, Lan received a corresponding chip, allowing MegaMan to freely access it when needed.

While it is Double Soul that overrides MegaMan's Style Change option in the games, it is a side effect of Cross Fusion that serves this purpose in the anime.

All twelve Souls from Mega Man Battle Network 4 appear in Axess. Only one Soul from Mega Man Battle Network 5 (Gyro Soul) appears in the Stream season.

List of Double Souls
Below is the list of all Double Souls appearing in the games and anime.

Trivia

 * Mega Man Battle Network 4 contains unused data of a Forte Soul (that name is also in the English version) and Duo Soul, but they can not be accessed with hacking due to the low amount of data left.