Keeper is able to pick up items independently from Isaac. He has his own inventory.
This inventory is not displayed on the HUD.
When Keeper dies, the items he obtained are permanently removed.
Keeper is not able to pick up any active items, trinkets or consumables.
Keeper cannot pick up any type of hearts, except for Golden Hearts.
Eternal Hearts can also be picked up, but to no effect, effectively wasting them.
Keeper's speed is independent from Isaac's and does not sync like with Jacob and Esau. Obtaining speed items will grant speed only to the character that picked up the item.
Devil deals will cost money instead of health as long as Keeper is alive.
When placing a bomb, Keeper will place another at no cost.
Keeper can be used to play Sacrifice Rooms at the cost of coins instead of hearts.
Keeper is not able to walk through doors or crawlspaces, although he is able to enter the beam of light that takes Isaac to the next floor after the Mom's Heart/It Lives! fight, the one spawned by Genesis, as well as entering the big chest after defeating bosses in later stages.
Keeper is able to obtain items that grant extra lives and use them normally. If one of these items transforms the character into another character upon death, Keeper will respawn as that character and remain controllable.
The blue spiders will not spawn if the Keeper respawned with a respawn item.
Similar to Jacob and Esau, it is possible to take both items between item choice pedestals with careful positioning.
Even though Keeper is considered a second character, the run ends if the original character dies.
Technically, Strawman is marked as a familiar item in the game's files, although familiar-modifying items, such as BFFS! don't have any effect on Keeper.
Unlike Jacob and Esau, invincibility frames are not shared between the characters.
If collected in a challenge where Isaac is blindfolded, Keeper will be blindfolded too.
Crooked Penny/Diplopia: Picking up multiple copies of the item will spawn additional Keepers that each have their own inventory.
Extension Cord: Beams of electricity flow between Keeper and the original character.
Genesis: Will not grant item choices for any of Keeper's collected items.
A straw man (sometimes written as strawman) is a form of argument and an informal fallacy of having the impression of refuting an argument, whereas the real subject of the argument was not addressed or refuted, but instead replaced with a false one. One who engages in this fallacy is said to be "attacking a straw man".
Picking up Strawman while having Money = Power causes the game to crash. If the order is reversed, the combination works normally.