@VibrantFox nice. Can't wait to see it!
@jensrichard77 that makes SO much sense!
Jake would be an oboe - he shines the best when people know him really well, can be underutilized, but also slightly melancholy 
Damien would be a cello - melodic, brings everyone together, never intentionally overpowers anyone else, and gets along with anyone 
I feel like Jaime would probably be a collection of soft orchestral backing, mostly string, rising and falling with his current mood. Maybe a nice piano leitmotif that runs along in the background every once in a while as an accent. Pretty big Dear, Evan Hansen vibes.
Jade is pretty bombastic. I wanna say a pretty consistent electric guitar, but played mostly with low/mellow notes that pretty much constantly jams out into nice loud chords of a higher octave. Again, mostly set to individual scenes and such.
I love thinking of associated stuff my characters all the time! Sadly, I am musically inept, so I have no avenues to pursue in regards to character themes. XD
Not to knowledgable with instruments so hope these make sense haha
Feel like Dry would be an electric guitar. Showy, and people like it's one of the main stars of a song.
I imagine Spring would be a violin. Something that people think of when it comes to classical and proper instruments. Also melodic and harmonious.
Rainy would probably be a drum set. Maybe not as popular as other instruments, but still important. Can be steadily be played in the background, or be loud and thunderous.
Autumn would be a fiddle. Playful and down to earth.
Winter I think would be a flute. Playful and energetic.
Summer I think would be a drum. Tries to keep the other instruments in rhythm. But considered a fun instrument.
@alliascompany I think those all sound just like them xD nailed it.
I think my main character would be a piano. She's a "jack of all trades" sort of person, not perfect just somewhat competent in different tasks as long as it's not making props. Pianos are versatile. They could be classical, country, ballad, rock, etc. It can be use to create compositions before assigning the other instruments. Just like a piano, when treated well she shows a happy side. Otherwise, the piano can't play.
I also have a couple main characters that i can't reveal yet: One has a happy-go-lucky and mischievous personality, and other has a graceful, poised and calm demeanor.
Cello (comedy pizzicato style)
Harp
I'll do another of my characters.
Freya
Freya would be a lead electric guitar. She's fast. Flashy. And demands all of the attention with her attitude.
The licks and runs from a beastly shred solo call to mind the arcs and crackles of her power Lightning Reflexes as it courses through her muscles, giving her the speed of a lightning bolt.
Now this is an interesting take on these kinds of threads, I love it 
After a looot of thinking I reckon my MC, Aina would be a hurdy gurdy1 or a nyckelharpa1. Both are instruments that have a "drone string", which plays a single note/chord through the whole piece independent of other strings and represents her constant thinking in the background of everything. The instruments are also both tied to folk tradition/lore, which the character herself is very immersed in.
@BlunderingAlbatross yeah I had this thread idea for about a year but was too lazy until the other day to pull the trigger XD
Got sidetracked but about the notes: I'd remembered having this idea when I listened to Avi Tran and Sura from the Crewniverse talk about the instruments/sounds associated with characters in the shows cast. I'd say that was the first thing to inspire me with such a thing but actually goes back to "Peter and the Wolf" where each of the characters in the musical (?) were represented by an instrument. I loved seeing it on PBS because it just seemed so clever and not only did the characters personality shine but that of the instrument.
So getting back on track to my own characters I've only really thought of it in abstract and ironically while brainstorming for this thread I'd thrown together a random little song thing related to characters of one of my WIP stories. That being said I think these instruments correspond to some of my most frequently explored characters.
Kiren - Electric or Acoustic guitar. Both though more the former can be really powerful and leads well as does Kiren but can also be pretty wild and I think it suits his calm but sometimes fierce personality.
Socket - Synth piano. It's got a fun playful kind of sound to it but if played right can also produce more mellow sounds which is 100% on key with how she is.
Rye and Haku - both are in the chiptune/8-bit digital instrument category and whats great about it is the variety of tone. Haku's tending towards more bright and playful tones versus Rye's being a bit more wild and in some cases kinda crazy or strange (think Earhtbound)
Aileen - Something like a kalimba or 8-bit. It's got a very cute sound and just makes me thing of the curiosity of youth and the feeling of looking at things with new eyes which is a big part of their personality. Which isn't to say they don't have their sassy or spunky moments so the funky side of 8-bit would fit her well.
Lucid - definitely Piano. Loud and powerful when it needs to be but also soft and delicate and she's very much both, being a commanding but also very caring.
Nona - DRUMS! The can be quiet they can be loud and Nona 100% goes to the beat of their own. They're actually kind of a vent oc so the point was to let them be loud in places where I was quiet, so it fits
Max - Modular synth. Not sure if it counts as an instrument but anytime I think about him it's eurorack and synthwave stuff coming out of one of those bad boys. He's got a very cool demeanor but can be a force to be reckoned with if you push the wrong buttons.
Vi and Mono are just variants of myself so picking an instrument its tough but in terms of sound it's honestly pretty broad. If anything it'd probably be a mix of all of the above given my characters all have little fragments of myself mixed in so to pick just one wouldn't work ^^;;
Honestly though I've got a lot more than just these but they're the main folks from my current comics Foxtales and Gekkering so I figured I'd stick to those 
Epic. You've really put a lot of thought into it. And I remember that Peter and the Wolf cartoon. As a kid I actually had an audio version of it on a cassette tape.
Thinking back, that is probably where I started connecting characters and creations to music. Storytelling for me is so intertwined with music I couldn't work without it.