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 both versions of Mega Man Battle Network 5. It replaced MegaMan's Style Change capability of previous iterations, and is itself replaced by the Cross System in Mega Man Battle Network 6.

Double Soul may also have been the basis for the Trans Arm system from Rockman.EXE Legend of Network.

Description
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).

Once the player has gone through the Double Soul tutorial, a "Unite" button appears under the "OK" button in the Custom Screen, and lights up whenever the player has previously selected a Battle Chip that is eligible for initiating the Soul. Initiating the Soul will sacrifice the Battle Chip and render it unusable for the rest of the battle, while also taking up a slot for selection for that turn (thus limiting the player to 4 chips for selection if he chooses to initiate a Soul at the same time). Souls last for 3 turns (including the one where they were initiated) by default, though Battle Network 5 introduced NaviCust programs that can prolong the Soul's duration.

Chaos Unison
Introduced in Mega Man Battle Network 5, Chaos Unison occurs when sacrificing a Dark Chip. It is an immensely powerful transformation, having the attributes of both the power of the Dark Chip and Soul Unison. MegaMan gains the Soul's attributes, with an unusual Double Soul different color (usually turns purple or blackish). The charge attack now becomes the same as the sacrificed Dark 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 rapidly transition between Teal-Green to Purple. If the player releases the Charge Shot while it is purple, the attack will execute flawlessly; if they release while it is teal-green, the attack fails and the Soul expires. Dark MegaMan is also spawned in the enemy area, and will proceed to harass the player with Buster shots and random chip attacks for 13 seconds or until the battle ends. For each consecutive time a Chaos Unison's Charge Shot is used successfully during the battle, its charge time will increase slightly, in an attempt to throw off the player's timing and raise the risk of failing it.

At the end of Megaman Battle Network 5, the destruction of the Dark Chip factory following Nebula Grey's defeat renders Chaos Unison impossible with no Dark Chips to sacrifice.

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

Other Appearances
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.

Manga
The secret Bass Soul and Laser Soul appeared in the NT Warrior manga, but used by Dark MegaMan.EXE instead of MegaMan.

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.
 * The time period that Dark MegaMan appears following the fail of Chaos Unison charge shot is 13 seconds, a rather unusual nod to an unlucky number.