Isaac turns into a walking Mushroom after picking up three distinct items from the following list. Grants a Red Heart container.
||Gives an extra life. Upon death, you will respawn in the previous room with full health and lose the 1up!.
||HP + tears up + shot speed down
||Grants 1 Red Heart container and increases tears, but decreases shot speed.
||All Stats Up!
||Increases most stats, enlarges Isaac, grants a heart container and restores all health.
||Speed + range up
||Increases Speed and Shot Height and decreases Isaac's size, but lowers Range.
||Grants an empty heart container. Increases damage, range and shot height while reducing speed.
||Fire rate up
||+1.7 tears, +0.3 speed, reduces damage.
| God's Flesh
||Tears now have a random chance to apply a shrinking effect, causing enemies to shrink in size and also run away from Isaac. Shrunk enemies can be crushed and killed by walking over them.
- The name Fun Guy is a slang term for psychedelic mushrooms and a wordplay on fungi.
- Fun Guy may be a reference to the character of the same name in Chrono Cross.
- Fun Guy"s appearance may be a reference to the character Toad, a humanoid mushroom from the Mario franchise.