Final answer:
The familiar in The Binding of Isaac: Rebirth that shoots charged brimstone shots is Lil' Brimstone, and is found in the devil room. It is a valuable companion for area damage, differing from the abilities of Demon Baby, Sister Maggy, and Dark Bum.
Step-by-step explanation:
In the game The Binding of Isaac: Rebirth, the familiar that follows Isaac and can shoot charged brimstone shots, similar to the normal Brimstone item but inflicting less damage, is found in the devil room and is known as Lil' Brimstone. This item is an excellent addition for players seeking a companion that can deal substantial area damage, mirroring the capability of Isaac's own Brimstone item, but as a companion. The other options, such as Demon Baby, Sister Maggy, and Dark Bum, offer different abilities and do not possess the trait of shooting charged brimstone shots like Lil' Brimstone does.