Prisca, Dawa, Renfield = rubbish without costumes. Greymane also not up to much. Others may feel differently but I’ve never levelled Friar Tuck even though I do have his costume. Maybe one day - or maybe never.
Nobody needs more than one Nashgar (but one is good).
Brienne is great but you probably don’t need more than one. If you get her costume you will get another Brienne with it anyway, and then maybe keep one with costume and one without.
As you get more healers you’ll probably decide that one Hawkmoon is enough. She’s another with a good costume so if you get that you may want to go back to having two - or maybe not.
I’d keep one spare Kvasir (indeed I have!) but you probably don’t need more than two.
Also consider how much you’re going to use Jafar and Hisan. I know it feels bad eating event heroes because they’re so hard to get, so maybe hold onto the spares for now (and then lose patience and eat them later on).
Personally I like to keep one of each hero just because - although I didn’t keep Prisca, Dawa or Renfield until I had their costumes, and even then Renfield is the only one that’s really good. Bear in mind that any portal you summon from is very likely to give you more s1 rares than anything else, so you won’t regret it too much if you fail to keep one as there’ll be another along shortly… and another… and another…
Something I was told early on in this game is that it makes more sense to spend gems on increasing roster space than on summons. You’ll want space not just for those shiny new heroes that will almost certainly come your way eventually, but for the feeders to do on-colour ten-at-a-time levelling of said shinies.
Good luck and keep on slogging 
