|The following is based on Fallout Tactics and is not canon.|
In this special encounter you find the famous Vault Boy, called Pipboy here, wandering around a radioactive playground. He speaks in a strange 50's style and carries an FN FAL. If you have a full squad, he wanders away when you talk to him. If you have less than that, he joins you.
Behind the scenes
As explained in this article, the "Vault-Boy" is incorrectly called "Pip-Boy" in this encounter - "Vault-Boy" being the name given by its creator, Leonard Boyarsky. It is a common misconception, as the character was never given a name in-game, only referred to in interviews.
He may not join you if your squad has more than two characters.