I would prefer Desmond. Here's why:
His lv.1 Super would summon Altair
His.lv.2 Super would summon Ezio.
His.lv.3 Super would summon Connor.
Each Assassin would aid you, and have unique attacks like an AI.
Aveline could be the minion (or Lucy I guess.)
Costume 1: Desmond.
Costume 2: Generic Assassin clothes.
Costume 3: Abstergo Industries lab coat.
Attacks: Put some Apple of Eden moves, hidden punches, hidden blades(although I don't think Desmond has one), Swords, etc. Eagle Vision could be use to briefly see how much AP someone has(an actual number.) Leap of Faith could be a move where you dive into a random haystack and hurt on your way down and can stay in it until someone hits you.
Lv.3 could also be Desmond going into the Animus and everyone are Templars and each time you are a different ancestor