Mercs character list:
Chris (both flashback and current)
Leon (both outfits)
Piers (same)
Helena (same)
Sherry (same)
Jake (same)
Ada (and Carla as a bonus)
Carla (her original face, with her own weapon loadout)
HUNK (modified RE2 uniform to look more like in Operation Raccoon City)
Wesker (ported from RE5, no differences except in loadout)
Barry (in BSAA gear but nothing changed except loadout)
Different costumes will have the same loadout as the original character. Loadouts are now customizable but you have to level up as that character first. Experience is earned universally on all the modes but is seperated by characters, costumes are unlockable. You can also purchase to unlock all the costumes instantly and it'lll instantly move you up 10 "levels" (levels aren't numbered but are determined by BSAA rank, ex. Recruit, SGT, etc.)