Slay the Spire’s fourth character — the Watcher — is distinguished from the three original characters by one mechanical property: she can see the future. In her unique stances, her block and attack values are fixed rather than probabilistic. She does not gamble; she has already seen the outcomes.
The Watcher also unlocks the fourth act — a final area beyond the three-act structure available to other characters from the start. The fourth act requires specific conditions: defeating the three act bosses without dying in a single run and delivering a specific item to the Spire’s crown. The Watcher does not need this item delivered; she can reach the fourth act through a different path that the game does not explicitly describe.
MegaCrit built the Watcher’s relationship to the fourth act as the culmination of her character: the character who sees the future is the one most equipped to navigate an area that other characters cannot reach reliably. Her mechanical advantage and her narrative arc converge at the final act in a way the game presents as inevitable rather than designed.
The detail work the devs put into areas most players never visit is what separates great games from good ones.
I’ve put 200 hours into this game and never caught this. The developers really reward the obsessive players.