Spoiler If Fi Could Fight...

[Kupsacker]

I don't exactly think I fight between the two would seem... natural. I would've liked to see more interaction between the two though, like Fi beating up Ghirahim with intellectual insults would be hilarious.

 

[Feral]

Actually if Fi does fight I would also imagine she would fight relentlessly and emotionless under her master's command like a robot....maybe terminator style.