Fin actually did have three squad mates that he was trained with from birth. I think their names were Nines, Zero and Slips. With the first two being their identification numbers and Slips being called so because he kept screwing up. Fin himself didn't have a nickname at point, obviously.

Anyway, it's possible that TR-8R is actually Nines or Zero, assuming that Slips was the stormtrooper killed at the beginning of the movie that causes Fin to abandon the first order.

I don't think he's coming back, but that still leaves us with one more squadmate, so get hype, I guess.