From Diablo Wiki
Remoras are small parasitic fish that attach themselves to larger fish, usually predators like sharks or mantas. The remora uses a sucker on its head to stick to the larger fish, which tows it around. Remoras do not hunt for themselves, but instead scavenge scraps from the larger fish's kills, or actually consume the feces of the host fish.
Remoras in Diablo
The term is used in Diablo II to refer to support characters who can not kill for themselves, and who tend to follow more powerful characters around, sharing experience from kills and occasionally darting in to steal good drops while the more powerful character is busy killing the demons.
Like the aquatic type of remora, such characters may be more symbiotic than parasitic; lending auras or curses to improve the killing power of their shark, but like real remoras they are essentially helpless on their own, and must rely on the work of larger predators in order to survive.