Erthor

Erthor, much like the brothers of Weatherleah, is a capable fighter and will do exactly as you say no matter his disposition. He goes missing and you are required to find him as part of the Skingrad Recommendation of the Mages Guild. If you are not as interested in receiving the recommendation, you can utilize him as a "gang member". He will follow you into all areas of the game and cannot die. He will fall unconscious after receiving a substantial amount of damage, but will always show up a few minutes later or after a load screen. He will also (sometimes) follow you in Oblivion Gates and will strike first at any enemy.

You can also tell him to wait anywhere, so leaving him in your home or favorite inn for later use is quite handy. You may even pickpocket him.

However, after a substantial amount of time abusing his uses, he will grow weary. Any attempt to talk him will respond him saying "I've had enough of the wilderness for awhile." He will then turn you in to the guards if you attempt to steal anything within his sight. He also becomes quite lazy in defending you, and will go against you if the guards have reason to attack you.

Still, you can use him for many game hours before this, and can profit nicely, especially combined with the Weatherleah brothers on terms of raiding forts or closing Oblivion Gates.