Also the format isn't set in stone. I currently have two spirits, but will pick up a third later.
Let's discuss spirit abilities. One idea I have is Reyn channeling upper body strength to Link to make it easier for him to pull himself back up from a cliff. May work better with Shulk, since Link is more of a wilderness guy, but yeah, spirits can help fighters they don't know all that well too.
Spirit skills for my two and eventual third may be something like:
Fiora's Swift Thorn: A lightning fast dodge and counterattack.
Reyn's Muscle Boost: A burst of strength.
Dunban's Speed Blade: A flurry of attacks.