Pipboy is an actual person version of Vault Boy, who could be encountered by the Warrior as a joke character. He may join the Brotherhood of Steel and fight as a squad member, but is not considered an official member by the Brotherhood, due to not being a normal person (which in this case is to say he does not fit into any known species of the wasteland).
Pipboy becomes available after you find him in the Pipboy special encounter. He joins the group when you speak with him if you have fewer than 5 members. He will go away and appear in the recruits pool if you have 5 members. If you have 6 members, he will do nothing.
He will join you even if your level is lower than when you first find him.
He has a Perk rate of 3 at the time he is first discovered, and he is level 8 with 3 perks. This is likely due to the fact that PipBoy cannot change stance; by granting Flexible as a freebie, the player is protected from later wasting a gained perk on it.
He moves considerably slower than other characters when not in turn-based mode.
He has no biographical text in the character screen or in the recruitment pool.