Double Soul

Double Soul, known in Japan as Soul Unison (ソウルユニゾン), is a term used in the MegaMan Battle Network series of video games and MegaMan NT Warrior (Rockman.EXE) anime and manga. It is an ability employed by MegaMan.EXE (Rockman) 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 MegaMan Battle Network 4. Out of twelve souls, six were exclusive to either game. It appears once more in MegaMan Battle Network 5: Team Colonel and MegaMan Battle Network 5: Team ProtoMan. It replaced MegaMan's Style Change capability of previous iterations.

General
In the closing events of MegaMan Battle Network 3, MegaMan.EXE sacrificed himself to save his operator, Lan Hikari (Netto Hikari) from absorption by the horrific Alpha (Proto) 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 your team (excluding ProtoMan/Colonel, which is gotten near the end of the game instead).

Chaos Unison
Introduced in MegaMan 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 1 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 you for about 13 seconds (a rather unusual nod to an unlucky number) or until you beat the other viruses, randomly firing at you and using chips. The Chaos Unison is a immensely powerful result, having the attributes of both the power of the Dark Chip and Soul Unison.

Double Soul
In 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 MegaMan Battle Network 4 appear in Axess. Only one Soul from MegaMan Battle Network 5 (Gyro Soul) appears in the Stream season.

Cross System
Cross Beast makes during Beast+. The process occurs whenever Lan links the data of another Navi with the Beast Out file housed in his PET. As a result of Cross Beast, MegaMan not only becomes more powerful through the incorporation of another Navi's data into himself, he also takes on that Navi's personality; in the original version of the series, Rockman acts more childish after a Cross with AquaMan, and acts more arrogantly after a Cross with TomahawkMan (dropping the honorific of "-kun" when addressing his operator). MegaMan also displays the ability to perform regular Crosses as well through identical methods.

Fans
Most fans would often make their own double souls for other NetNavis like the Mega Soul, Glyde Soul, Pharaoh Soul, Bubble Soul, Skull Soul, etc.

Trivia

 * MegaMan Battle Network 4 contains unused data of a Bass Soul and Duo Soul, but they can't be accessed with hacking due to the low amount of data left.