If each game were to allow you to recruit one villain as a permanent party member, which villain would you pick? It can be any enemy other than the end boss, from the lowly rappy to a major mid-boss. You get to use all the abilities that enemy can typically use in battle.
My picks are:
PSI - Dr. Mad. Hey, he's got an instant kill attack against animals. That should be handy!
PSII - Neifirst. Offsets the loss of Nei, since she'd have mostly the same powers but more deadly.
PSIII - Those dinosaurs with the huge attack power. Gnashers, wasn't it?
PSIV - Lashiec. Bringing down those powerful attacks on the enemies would be very satisfying.