I disagree with Setsuna being the best mom for Selkie, probably mediocre at best. She passes down archer line with the only skill that's really notably useful is Prescient Victory (+4 damage when initiating battle). Her speed is already great, any more is overkill, and there's not really any merit in specializing in it since she will double nearly all enemies anyways. I'd say she benefits way more with Kagerou, Oboro, or Hinoka.
Kagerou really patches up her offense while giving her a great class tree, and you can do some sheninigans with four fangs + snake venoms, along with copycat.
Oboro gives great all around modifiers and growths, and allows for nasty enemy phase combos (four fangs + speed + defense seal).
Hinoka is just all around great for Selkie.