http://cdn.collider.com/wp-content/uploads/marc-singer-beastmaster.jpg, http://cdn.collider.com/wp-content/uploads/beastmaster-slice.jpg don't engage in battle but they are still cool to have along on an adventure as long as they become scarce when things get hairy and reappear again when they calm down.
Passive companions such as small mammals and birds would be pretty cool if they had abilities or special interaction with the environment.
I want one the ravens in the game as a passive companion that follows me around the wasteland and feasts on the eyes of my fallen enemies.