The Initiate can acquire a canine companion named Dogmeat by getting the skill Man's Best Friend (skills in this game work like perks do in other Fallout games) and it will appear once the Initiate reaches a save point. If Dogmeat dies, he will reappear once the Initiate gets to another save point.
Rhombus will breathly help the Initiate get to the truck where he has hidden a keycard wich gives acess to the Secret Vault's Laboratories. When they get to the truck Rhombus is blown up by a ghoul suicide bomber and cannot go any further. He wears a Lightweight power armor and uses a Desert Eagle.
In Fallout: Brotherhood of Steel, the player can play with another person using another controller connected to the system. The second player would control another character wich can help in battles and later on the game can help acess areas wich require two players. Like Dogmeat, if the second or, also in this case, first player dies the other player can make them reappear once he or she reaches a savepoint.