Fallout: Brotherhood of Steel companions

Dogmeat
The Initiate, with the exception of Cain, 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), which will appear once the Initiate reaches a save point. If Dogmeat dies, he will reappear once the Initiate gets to another save point.

Rhombus
Rhombus will briefly help the Initiate get to the truck where he has hidden a keycard, which gives access 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.

Mary
Mary will help the Initiate briefly to guide them in the Secret Vault level.

Tesla robot
There are some Mr. Buddy consoles in the Secret Vault Labotary 3 which could be used to buy three special Tesla robots. These robots could be used against super mutants by the Initiate.

Multiplayer
In Fallout: Brotherhood of Steel, one can play with another person using another controller connected to the system. The second player would control another character, which can help in battles and later in the game, can help access areas which require two players.

Like Dogmeat, if the second or, also in this case, first player character dies, the other player can make them reappear once they reach a savepoint.