Offense
Reaper: Uses duel wield shotguns. Physically cannot reload and must throw shotguns down and pull out new ones from... somewhere. Ultimate: Spins in a cricle, shoots a lot, gets kills, yells "Die , Die, Die" 
Tracer: Poster child of Overwatch. Uses dual wield autmoatic pistols. Can teleport and go back to where she was three seconds ago. Ultimate: Throws bomb that kills everything in a certain radius. She's also British
Defense
Bastion: Killer robot that likes birds. Can turn into an immovable sentry that does super damage and can heal himself. Ultimate: Can turn into a tank, drive around, and blow stuff up. 
Mei: Cancer in female eskimo form. Freezes everyone, can build an ice wall in a second and encase herself in ice making her immune to damage and heals her. Ultimate: Sends her pet robot ice machine to create a circle of freezing death, killing and freezing everyone in a radius.
Tank
D.va: Just a girl in a mechsuit. Mechsuit has small twin shotguns with infinite ammo, a shield, and can fly. Outside of mech, she has a pistol and her ultimate changes to calling down a new mech. Ultimate: Sets mech to self-destruct, killing everything in a large radius when it goes off. 
Reinhardt: Old man in a high tech suit of armor with a hammer. Has a very large shield which absorbs a lot of damage, can swing a projectile out from his hammer, and rockets forward pinning down anyone in his way. Ultimate: Smashes hammer on ground, everything before him falls.
Healers
Lucio: Uses phat beatsies to do stuff and skates around. Can switch from healing mode to speed mode. In speed mode can make others and himself faster. In healing mode can heal everyone and himself in a certain radius. Can bop enemies back. Ultimate: The beat drops and everyone around him gets a ton of armor that drains fast. 
Mercy: Cancer in angel form. Flies to team members, has a staff that can heal people and buff them, also has incredibly weak pistol. Ultimate: "Heroes Never Die" the source of her cancer brings everyone around her who has died back from the dead with full heath.