Isaac's dead cat, Guppy, surrounded by flies.
Isaac's max health is set to one Heart Container. You are followed by a floating cat head and have nine lives (these still count as deaths on the statistics screen). You can increase your max health as normal, but after each death it will reset to one Heart Container again. (One Soul Heart with ???) The floating cat head will disappear when you use up your last extra life.
Even if you have more than three Heart Containers, you will only have one Heart Container left once you purchase Guppy, since it sets your max health to one Heart Container immediately, not after your first death. If you buy him for the price of two Heart Containers from the devil and you have two or fewer Heart Containers, you will acquire him, die, and respawn with one Heart Container, therefore losing one of the extra lives. If you have no health at all, you will get one Heart Container that is filled half. The cost of purchasing items in the Devil Room must be met, but if it would kill you, you will respawn.
It is possible to pick up two Dead Cats in a Curse Room by opening a Red Chest, not picking up Dead Cat, and then opening the other Red Chest. Picking up a second Dead Cat will reset Isaac's lives back to 9 and reset heart containers to 1.
- Guppy's Paw - Dead Cat has good synergy with Guppy's Paw, as each life effectively means three spirit hearts for a total of twenty-seven spirit hearts. As such it is a good idea to hang onto Guppy's paw if Dead Cat is already collected.
- After defeating a boss, Isaac can kill himself with a Bomb or something similar, respawn outside the Boss Room, and fight the boss again for another item. Items that appear after defeating the boss can either be picked up or left in place (for items that increase heart containers it is generally better to leave them in place, since heart containers are reset to one on death by Dead Cat, and you can pick up all of these items at once after you are done). This can be repeated as long as you have enough lives.
- Sometimes changes followers (such as Sister Maggy) into Brother Bobby. If this happens, the follower is corrected upon moving to another level.