okay, i have read everything !! i got an explosion of ideas for a possible character, but i'm not sure if shapeshifting is allowed in this magic system, so let me know if i can go with this since i can very easily come up with something different ;;
i was thinking of having a leviathan-human character. mom was a leviathan, dad was a human given a shapeshifting "curse" where he would be forever able to go everywhere but never fit in anywhere, but he kind of liked it for the novelty. fell in love with a sea monstergirl (strike "girl," kind of just a monster) and had a kid while in leviathan form that was this guy.
as a result, character's main form is a leviathan, he can turn into a human but the farther he shifts from his main form, the more energy he consumes and he also consumes energy simply by maintaining it. as a result he kind of just maintains a partially monsterous human form, mostly because it's less energy than maintaining a fully human form and partially because he just likes it that way.
he doesn't have too sad of a backstory. his dad was a human with a limited lifespan so he's dead now, but he remembers being with his dad, protected by a dragon, being carried by an eagle looking down at the view below, walking on land with a human and feeling the sand below his feet, and he misses the chance to do that, since his mom's just a sea monster. loves his mama but wants more. and she's gonna live to like thousands years old anyway so he says he'll just be out for a short while and fucks off to go chase a novel life like the one he lived with his pop.
he chooses the pirate life because it's as close to the sea as possible, meaning he still is close to his old life and if he's feeling dried out he can pop in for a quick swim, while still being majorly above sea level. and he thinks everything about it is so novel, loves everything about life on "land" (he's on a boat so not really but he's happy), appreciates life so much. he's kind of like a disney princess with his desire to experience something new if a disney princess were a 7' tall hulking scaly scare of a man, standing on the railing leaning out over the ocean looking out at the sky like he's about to burst out into his "i want" song. boisterous man, he thinks everything around him is so new and loves his crew for how they all present novelty into his life, though he can be kind of unintentionally cruel because of a shit understanding of human morality.
i'd think that he really needs to be fed a lot in order to maintain his current form lest the crew wants him to suddenly turn into a giant sea monster flopping on the deck. he looks real pathetic like that and it's happened at least once before on a different ship and he crushed the whole thing, so they know better than to let it happen. but he's generally useful since he lived in the ocean before this and understands the body language of sea creatures. he's small for a leviathan and huge for a human, likes to claim he's the king of the ocean by species but amongst his kind he's kind of tiny, ESPECIALLY since by leviathan standards he's just a teeny baby (he'd probably grow to be larger in a couple hundred years give or take). when he's in his leviathan form he's still about the size of the ship itself, so.
let me know if i should change anything. assuming all of this is okay with all of you, i think that it's been a while since he left home. he's a pretty quick learner so by this point combined with his monsterous strength, maybe he'd be gunner. or boatswain since that also requires a lot of physical strength.