Terai

is a member of the disbanded faction of Konohagakure's ANBU: Root.

Background
He, like most members of the group was likely adopted into the organization as a young orphan and trained to lose emotions, in order to better accomplish missions. As Root members each uses a codename, it is unknown what Terai's real name is, if he even has one. Even though the organisation has been officially disbanded, Terai remains loyal to its head, Danzō.

Five Kage Summit Arc
Danzō gave him the mission to find Kabuto before Anko, who was doing the same at that point in time.