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 ???)
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. ???'s health will be set to Soul Hearts. 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.
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, if you die after the item appears but before the boss disappears you will be able to fight the boss a second time for yet another item. (Pick up the item, then kill yourself with a bomb or something similar. You will then respawn outside the Boss Room with one less life on your counter. Going back inside the Boss Room allows you to now fight the boss again for another item. Can be done as long as you have additional lives on your counter. Can also be performed after picking up a 1UP.)
Guppy will sometimes appear as a collectable item as well as in the Devil Room. Collecting more than one Guppy will not add any more lives if the player already has nine, but will reset your max health back to one Heart Container.
When you get additional Heart Containers, your Heart Containers will reset to one after your first death.