Reinhardt

-res is ok I guess, since he usually gets doubled and dies by most mages anyway, so you always want to keep him out of harm's way and use him purely as a player phase unit. As long as it's your only Reinhardt then I would invest in him, because he brings just too much value to the team with death blow + hone atk/cavalry

So long as it's not a -Atk Reinhardt, I think he's worth investing in. The other stats don't really matter. +Atk is ideal, but as someone who has a completely neutral Reinhardt, he still ORKOs most everything he needs to.

As long as the IV isn't -Atk (which optimistically you want as the boon 100% of the time) ANY Reinhardt is perfectly viable. Having Res as your bane - that is to say not Def or HP - still leaves his reasonable Physical Defensive Bulk still intact, which (especially if boosted by a Fortify if running Cavalry) still allows him to wall versus sword users. If you're using Vantage or a similar low HP skill then walling suddenly becomes a legitimate (if unconventional) strat for the Blue Bastard.