A passerby because the other propositions won't work.
Things like advisor friend, supporting character etc I don't have space for one or would need to change the story a lot.
Couch potato or pet would not bring anything to the story, so what the point?
MC love interest, well, he's a teen and I'm older than his parents. NO!
A passerby is nice, it does not impact the story, it does not add unnecessary material, but it has a fun easter egg facet, actually I may do that, that'd be fun.