How Rebirth's aggro system actually works.
It has nothing to do with what character you're controlling. The enemies don't focus on only your playable character just because you're in control of them.
It comes down to a few things.
1.Who's attacking the enemy the most. (Which is shown in the first video)
RNG (which is shown in my 2nd video but I'm unable to share more than one video on Reddit) it's Cloud attacking the most against Kujata in Bonds of friendship, but Kujata targets Zack multiple times.
In a story boss fight the first enemy attack will target the party leader.
Phase 2 the first enemy attack will always target the character you start that phase with. (Everything else in-between is down to points 1 and 2 mentioned above)
Very interesting stuff and it can really give you an upper hand in fights if you know how the aggro system works.