One of the main characters at least has a close partenarship (may they be friends, lovers, master-student or parent-child) with members of another classical occidental fantasy race (humans, elves, dwarves, hobbits, gnomes, sprites, pixies, unicorns, dragons, trolls, goblins, orcs, merfolk, selkies, etc).

One way or another, their close relationship has played an important part in that character's destiny.
The character doesn't have to be human.

Not qualifiable are:
revenants (vampires, zombies, wraiths, etc),
shifters (werewolves, etc),
creature coming from another plan of existance (sylphs, ghosts, angels),
or creature from easter folklore sush as genies, djinns, youkais, kamis, onis, kappas, etc.