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.
Plankton voice Do instruments of torture count?
Just kidding! Steppe would definitely be an electric guitar. (I've sat there and thought about the fact that if he was a human, he'd definitely be in a heavy metal band.) Stellar would be a harp.
For sure! and that's pretty cool about peter and the wolf. I can totally agree on the way music tends to feel entwined with storytelling since it's a great way to amplify the mood aside from color/lighting schemes and visual framing.
For Will, I'd have to say the bass in something like this. He's always mentally present yet is not one to take the wheel and drive the conversation. Despite this, he's often the one to set / change the tone of the conversation due to his detached insensitivity and lack of restraint in regards to his thoughts and feelings.
Maybe a better way of putting it is - he is a bass that occasionally hiccups and becomes a dissonant guitar, yet then returns to being a bass right after.
Oh that is a hard one... But if I had to pick
Rokuzo: A piano because there is so many genres you can play, and has a deep history. Its complex, beautiful, emotional, technical, and in the right context elegantly powerful.
Ahiko: A clarinet, because they are often underrated. But, they are actually a very nice sounding instrument and know for their steady tone. consistent and reliable tones (thanks Squidward, to bane the existence of clarinets. We don't sound like that and if you do, you are either inexperienced, or have a broken reed and/or instrument)
Enossa: I want to pick a percussion instrument because the percussion helps keep the beat, but each instrument has its only qualities. I would pick either the quads or timpani. I like the tonal variation of the instruments, and their tones are not as ridged as others percussion such as the base drum, but are still equally stable and demanding.
Spider Lilly: A trumpet who always demands attention, and very loud.
My main Dark-Fire would be a bass drum because he is moody.
Hana/Hope would be a flute because of her nice and quiet personalty.
Noise would be a piano because throughout the series she is intense and sad.
Harmony would be a clarinet because of his relationship with Dark and Hana.
The Hooded man would be an organ because of his role in the series.
Riley would be a flute because it gives off a whimsical and gentle sound and Aaron would be a saxophone because it gives off a suave and emotionally powerful sound. So Riley's a gentle person who's got a childlike sense of wonder while Aaron's an elegant person who's emotionally strong.
Here are videos showcasing those instruments:
1 month later
So my mains are two characters but imma focus on the one who's more important to the plot, but I might talk about the other one later so who knooows~
Haruka's easily a classical guitar.
Quite frankly, when I think of the instrument, I tend to only think of somber or tense pieces of music. The way people tend to play it too is different from regular guitar as well, in a way that almost forces them to become better at this specific variation of it. They essentially play both rhythm and lead. The piano of guitars so to speak. Even design-wise it's set apart from the other versions of guitar. Classical guitars tend to have wider necks and more spaced out frets which allow for more precise playing. The reason why I chose this over piano was out of the timbre comparisons.. A classical guitar will always sound warm, no matter what you play.
Haruka's a character built out of suffering and through that she's developed her fighting skills intensely for her job, almost needlessly risking her life in the process. As a result, she's got a very tense personality for first impressions, but underneath it all, she's very compassionate about people whether she believes it or not.
Here's what I think encompasses her as an instrument. The classical guitar is the backbone of the song here, but it's so rich in it's sound. I couldn't not share it lol
For a better showcase of the instrument on it's own, here's this.