Update, April 27: The third Slay the Spire character, the Defect, is now playable.
You can now play the third Slay the Spire character. The Defect, which became available earlier today, can be accessed via the game's beta version.
The Defect is a combat robot that has become self-aware. I've been playing as it over my lunch break today, and while it's complicated, it's easily the most interesting of the three characters so far. It works by channeling 'Orbs' which you can summon with your cards (although you start with one orb at the beginning of each run). You cna operate three orbs at once, and each one offers passive bonuses, such as providing Block or dealing damage to random enemies.