Fullbringer is a race and obtainable by talking to the Shamook Npc. Upon talking to this you will be granted with the quest "Kill 4 other mere humans" for this, you will have to find four other players who have yet to select a race (Lost souls).
Once you have completed this quest you go back to the quest giver (Shamook) and he will ask if you truly wish to become a fullbringer, saying yes will give you access to fullbringer abilities such as Bringer kick, Bringer Light ( flash-step for fullbringes), soul crush, soul reinforcement, and a couple more.
Obtaining fullbring
Obtaining fullbring is simple, and slightly time consuming. After a player reaches the required level (150) you must talk to Shamook once again, however this time, he will say you are experiencer enough to learn fullbring, to learn fullbring you must die by a stage one hollow (Fishbone) 5 Times.