chum

acquaintance

associate, cohort, colleague, companion, fellow, hearty, hobnobber, partner, peer, sport

blood brother, brother, main man, sister

abettor (also abetter), accomplice, ally, collaborator, confederate

pen pal

benefactor, supporter, sympathizer, well-wisher

friendly

affiliate, ally, attach, band, bond, club, collaborate, collude, confederate, conjoin, connect, cooperate, couple, gang, get along, get on, group, interrelate, join, knot, league, link, mingle, mix, rally, relate, side, socialize, team, tie, wed

befriend, friend