There's a pretty number of characters to play. After the tutorial get receive enough eternity splinters to unlock one character. You can change to any character at S.H.E.I.D. HQ and play as them, but you can't change out characters when you're out and about unless you've unlocked them. There aren't really any support characters in the traditional sense. I've only played as Captain Marvel so far, and she has a self-heal you can use after a short timer. I assume other characters have it as well. There are also items to can equip that will also heal others a bit as well, but I'm unclear if it is only for party member or anyone friendly within range.
There's a pretty good number of characters to play, some of which surprised us a bit, i.e. Angela (Chad was more surprised/impressed with this than I b/c I had no idea who she was. She was originally in Spawn if you are like me and unfamiliar with her) and Blade. It's a nice mix between characters seen on TV and in movies as well as those most people might not be as familiar with if they don't read comics.
Full list:
Angela
Ant-Man
Beast
Black Panther
Black Widow
Blade
Captain America
Captain Marvel
Colossus
Cyclops
Daredevil
Deadpool
Dr. Strange
Gambit
Ghost Rider
Hawkeye
Hulk
Iceman
Iron Fist
Iron Man
Jean Grey
Kitty Pryde
Luke Cage
Magik
Moon Knight
Nova
Psylocke
Punisher
Rocket Racoon
Scarlet Witch
She-Hulk
Spider-Man
Squirrel Girl
Starlord
Storm
Thor
War Machine
Wolverine
There's also multiple skins you can buy for everyone, though it does seem like they missed out on a couple skins that could have been great i.e. Captain Marvel and Storm. On the flip side, they also included skins for what people are used to seeing in current screen media as well as classic costumes (omg... Blade's original costume. Thank you Wesley Snipes for making it better).
I'm not really one for microtransactions, but if I enjoy playing them enough I can see myself buying skins for Squirrel Girl's current costume, for Old Man Logan, and possibly Angela.