The portraits hanging on the wall on the game’s main page represent all the characters you currently have unlocked.
If a portrait shows a ? sign it means a character is still locked for you.

To pick your character, just tap on your favorite portrait on the wall to open that character’s menu. Tap on the heart symbol below a character to mark it as your favorite for your next matches.

This screen is also where you can add different accessories and outfits to your character! Anything you have unlocked will be displayed here so you can equip it before joining matches.

During matches, you can only choose to play with the characters you have unlocked. However, if another player has already chosen that same character before you entered the match you may be assigned a different one.