[@LostDestiny] The way I picture it, Ishida had to consolidate Sarayashiki before he could dream of going after the Ikebukuro Diamonds. So if he was the leader of the student council faction, then to own the whole school he also had to conquer the turfs of the cheer squad, the kendo club, the baseball club, etc., and meld them under his command. I imagine all of us used to belong to different factions (possibly clubs, possibly just delinquent cliques), except for those of us new enough to the school that we don't remember how things were before. (Or who belonged to the winning side in the first place.)

That's my mental picture, anyway.

Yuuya used to be the bancho of the baseball team's faction before Ishida's people beat him. So even though he hasn't done anything wrong at the moment, there's the simple fact that he's a recently defeated enemy who may be harboring more than a few resentments. Imagine you're an ancient Roman legionnaire and your Centurion has just allowed a bunch of conquered Gauls to join your legion. It's perfectly natural for someone to think they haven't sufficiently proven their loyalty to deserve any trust.