The familiar dies after taking any damage, and will respawn on the next floor.
If Found Soul is dropped before its death animation finishes, it will still be available for the current floor if picked up again.
First time pickup of this trinket may result in it not scaling with Isaac's speed properly. Regardless of Isaac's current speed, Found Soul's speed will be at 1, which may lead to detrimental de-syncs. This issue is usually fixed by traveling to the next floor.
Found Soul is capable of copying the effects of most items, including but not limited to:
In addition to copying/doubling many passive items, Found Soul can utilize the same active item as Isaac, effectively doubling them. Items that are affected by Isaac's damage will scale with Found Soul's damage instead. These include, but are not limited to the following:
The text when picking up the trinket, "Finally!," is a reference to the "Finally, Upgrade" meme.
While Found Soul is alive, damage dealt to bosses is reduced by 20% in an inconsistent manner. Isaac's normal tears, Mom's Knife, Spirit Sword, familiars' attacks (including Found Soul) and contact damage are reduced, but many other attacks such as Brimstone and Dr. Fetus are not.
Resuming a Jacob and Esau run with Found Soul equipped in Esau's trinket slot will require a controller to continue. After plugging the controller, Found Soul can be controlled like a Co-op baby and won't mimic either brother's movements. This effect won't happen if Jacob holds the trinket.
When used together with Azazel's Stump, the soul familiar sometimes keeps Azazel's properties (flight and short brimstone) for a few rooms after Isaac has transformed back to normal. Lil Clot also has this bug.
The synergy with Purgatory can cause the game to crash if the soul familiar dies while the item effect is active.
If Sulfur has been used in a room before picking up Found Soul, it will retain Brimstone until dropped/killed, even through rooms.