At the end of the chamber, Ser Manuel fights the player as a final challenge.
Ser Manuel's attacks are very basic and easy to avoid, considering how he's the very first boss in the game.
- Fires a ring of 16 bullets outwards, followed by a second ring of bullets.
- Rapidly fires bullets directly at the player.
- Teleports to a random area in the room.
Ser Manuel may be encountered inside the Gungeon in a secret room. He will be wearing an owl costume and comment on how since the player got their revenge on Blockner, he can get revenge on the Gungeon. He will award the player with a random item.
If found in this manner, he will say "I've known you were something special ever since you killed that defenseless bullet, completely in cold blood, in the Halls of Knowledge" in reference to the tutorial.
- Ser Manuel claims to have woken up in a chest after being stabbed in the back by Blockner, a reference to the way players can revive their teammates in co-op mode by opening a chest, which contains the dead player.
- Ser Manuel's name, along with his in-game function, is likely a pun on "User Manual", or instruction booklet.
- Ser Manuel's skeleton can be found in the tutorial's secret room.
- If the player continues to shoot Ser Manuel after defeating him, he will tell the player to leave and pity him.
- The name Monster Manuel, which he refers to himself as when found in secret rooms, is either a reference to various games, such as Dungeons & Dragons, referring to bestiaries as "Monster Manuals", or to a typo in the original Binding of Isaac; the item Monster Manual had its name misspelled as Monster Manuel, and became a running joke within the community.
- During Ser Manuel's boss fight, he wields the unused Ser Manuel's Revolver.
- Beating Ser Manuel in the boss fight for the first time will reward the player 10 . Beating him without taking damage rewards 20 .
- Continuing to speak to Sir Manuel after defeating him, will unlock some otherwise hidden dialogue, until "Taking a vow of silence, and sobbing"