Fusion effects see plenty of use in anime (and... other shows...), but there's no formal Ability for it in OVA and it gets asked about (particularly in the context of giant mecha) a little bit, so I got to thinking. Here's how I'd run it.
A new Ability, Fusion! You may design your Fusion Ability out of any Abilities and Weaknesses your character already possesses, up to a total of your level in Fusion. When one or more characters want to fuse, all players should roll their Fusion dice and try to beat a DN equal to their combined level in Fusion. The characters fuse if all of them roll successfully. Otherwise, the fusion fails.
Some possible modifiers include --
- If the character is able to perform a full-round "Fusion Dance" without interruption.* +4
- If a fusion partner is the character's Love Interest. (A bonus equal to their level in Love Interest.)
- If a fusion partner is the character's Rival. (A penalty equal to their level in Rival.)
- If the characters have attempted fusion previously in the same combat, even if the fusion was successful. (-1 for each Fusion attempt.)
If the characters fuse successfully, the new Fusion Character has all the combined Abilities and Weaknesses from the component characters' Fusion Abilities. This way, each player gets to pick what is important about their character (or Vehicle!) to add. Fusion Characters can use any attack techniques from any character that contributed Attack, and players can collaborate to design attacks that are unique to their Fusion Character if they want to. (In between game sessions, please!) A Fusion Character gets a number of combat actions equal to the number of their component characters, and each of those turn's actions is controlled by a player who contributed a character to the Fusion. A Fusion Character has 40 base Health and Endurance, just like any other Heroic-level character. If the Fusion Character takes damage, that damage is equally divided among all component characters when they unfuse.
Yikes. They are really bad for each other.
If players can't compromise on opposing actions for a Fusion Character, they can roll opposing willpower rolls and the winner controls all of the Fusion Character's actions that turn. At the end of the turn, all players must make Fusion rolls again. If they fail, the Fusion Character unfuses and the component characters are Stunned for the next round.
Edit: Examples here.