i haven't read the novel but my theory is possibly because braun wears a suit and he's a host? and also i don't think our mc has made any other ghost friends other than braun at this point!
The good friend retual summons someone that has a connection to the item you burned. Braun is obviously very fancy and always wears fancy suits with a tie, so the ritual summoned Braun. If you think about it, Braun is one of the only ghost stories that has a tie (if not the only one) so soleum just got lucky with his item choice
The ritual summons someone random based on the item burning so I don't think Soleum summoned Braun because of prior interaction. But don't worry, Braun will be a very good friend (▰˘◡˘▰)
(at least in the beginning... ٩(๑❛ᴗ❛๑)۶)
Well he manages to accidentally summon another (and the last I guess) good friend, as for who, you'll find it out later.
Why a necktie would summon Brown?