The keeper revolves around money, unlocked with money, uses money as health, and has access to the shop with his special key, I figure money = power as a starting item makes the most sense for him.

TO INSTALL: Extract Keeper + Money = Power.rar to Users > (name) > Documents > My Games > Binding of Isaac Afterbirth+ Mods.
Run the game, then enable it in the mods section of the main menu.