The Wolves!
Arcturus
Arcturus Beaming - The Crane Wives
It's their namesake. Why would I not use it for their theme song? Other than that, it's hopeful and fits their (hidden) lore.
Kuiper
Call Down the Dragon - Lilith Max
It has a bunch of storm motifs, and this air of... powerful, yet sad, nobility. Which means it fits her quite well.
Airglow
The Woods - San Fermin
Now, this song is really interesting! When I first chose it, I thought it would fit her disappearance... But as the lore has developed, it shifted to fit another's, one that has affected Airglow quite deeply...
Polaris
We are Gods! We are Wolves! - Le Loup
I can't delve into why yet. Spoilers.
Loreley
Loreley - Blackmore's Night
Another namesake song, huh? Fits for a siren sea-spirit's daughter! (although I'm going to have to wait until the Murkwood seasonal rolls around to get her her fins)
Libra
Cheshire Kitten - S. J. Tucker
It fits quite well for the cryptid-like mirror-world explorer, doesn't it?
Tempest
Row - Sail North
The most metal, sea-shanty version of Row, Row Your Boat I've ever heard definitely fits this minor storm spirit. They also may or may not be named after the ship mentioned in it.
Chandra
Shut Eye - Stealing Sheep
It has this wandering-in-a-desert feel, which fits our former-traveler well! Or, maybe the feel's just due to the animatics.
Nietzsche
Look Away - The Dear Hunter
The villain gets a villain song! He also has another, but... no spoilers.
Lux
Rule #39 - Lore - Fish in a Birdcage
Song name's a bit confusing, since it includes a dash and I use the dashes to separate the song and band names, but it's Rule #39 - Lore, and it is perfect for a scholar such as herself.
Dusk
Metaphor - The Crane Wives
Spoilers. Also I don't know really why I chose the song... yet.