Dirty Fighter
Source Ultimate Campaign pg. 52 (Amazon), Second Darkness Player's Guide pg. 11 (Amazon), Advanced Player's Guide pg. 1 (Amazon)
Category Basic (Combat)You wouldn’t have lived to make it out of childhood without the aid of a sibling, friend, or companion you could always count on to distract your enemies long enough for you to do a little bit more damage than normal. That companion may be another PC or an NPC (who may even be recently departed from your side). When you hit a foe you are flanking, you deal 1 additional point of damage (this damage is added to your base damage, and is multiplied on a critical hit). This additional damage is a trait bonus.
Aside from the word "companion" appearing twice in the description, the phrasing "may be" rather than "must be" leans me more toward a Yes, but I'll drum for comment.
I also think Yes.
Rhe mention of the buddy looks like pure fluff to me, so it doesn't matter (mechanically) who it is, might as well be your childhood stuffed animal.
I'm pretty sure that trait applies no matter how you're flanking. The background simply describes how you came to be unusually good at exploiting flanks in general.
Its purely a story point, and a good one! So go with it.