Chester is mediocre at best. He’s ok in PvE but relatively useless in PvP.
Clyde is pretty great. Not the greatest plat but it does help land more crits if you’re piloting him. (AI misses too much to even notice a difference)
Callidus is meh. I don’t have too much experience with her but her usage is really low at the top level so I can only assume she’s underwhelming.
Fischer is pretty garbage. He’s generally considered one of the very low-tier heroes. I personally wouldn’t put any effort into him. His plat is good for PvE, but it doesn’t make up for his very poor performance otherwise.
Heroes that are good to bring to plat:
Nightingale, Dogface, Heimlock, Ifrit, Halo, Panzer, Ronin, Baron, and Mauler are all great options.