To date, the following players and character types are fairly confirmed:
Defender (melee) : Mike
Leader (melee) : Jesse
Striker (melee) : Nick
Unknown (ranged?) : Ben
In more universal MMO terminology, Defender corresponds roughly with Tank/Brick, Leader with Healer (except fun), Striker with (mostly single target) D.P.S., and Controller with (Area of Effect and Status inducing) D.P.S./utility.