My character's backstory is that he had to flee his city because he used a spell to save his best friend in the city watch. He is from a made-up city where spells and powerful magic were privileges that only nobility could have; it was illegal for commoners other than having simple magic items, like a broom that sweeps by itself.
My character had been studying in secret and is a level 3 fighter with Eldritch Knight archetype. Background: City Watch (from Sword Coast Adventurers Guide)
Note: this is actually a replacement character in case my main one dies or if I play a campaign that starts at level 3.
Is there a way to establish that his best friend ran away with my character and become a companion or something? Or, does that have to stay in my backstory?
While I realize that some responses will be "ask your DM" I'm looking for (1) if there is a way to do this and (2) if you have experience in making this work.
This is my third D&D character so I'm still new-ish to the game and I'm not entirely sure if this is feasible and/or practical.