Spirit
Spirits are enemies that only appear in 
Secret Rooms. They erratically dart around, attempting to avoid Isaac, while occasionally shooting a triple shot of oval-shaped projectiles at him. After 15 seconds have passed, Spirits will flee the room and disappear. Spirits do not need to be defeated to clear the room.
On death, Spirits spawn a Soul Heart.
Variants
Lifeseed Spirit
The Lifeseed Spirit is a unique variant of Spirit that only has a chance to appear from killing enemies while Isaac is holding 
Lifeblood Syringe. They will avoid Isaac similarly to Spirits, but will not retaliate with projectiles nor can they deal contact damage, and they will flee the room after 8 seconds have passed.
Lifeseed Spirits do not drop a Soul Heart on death, but will instead provide 1 point of charge to Lifeblood Syringe.
Trivia
- Spirits and Lifeseed Spirits are a reference to Hollow Knight.
- Both enemies are based off the Lifeseed. Lifeseeds provide Lifeblood Masks to the Knight when killed, which provide a similar function to Soul Hearts in The Binding of Isaac, granting additional temporary hit points.
- Additionally, Spirits appear to have a similar design to Primal Aspids, as both have the ability to fire 3 projectiles as an attack.
- Spirit's name is a reference to how Soul Hearts was sometimes incorrectly called "spirit hearts" by the community of The Binding of Isaac.