[@Aku the Samurai]: I have been summoned, and so I shall provide input for thee (because it's late and it'd be better to provide this statement than not).

My problem with using Ryujin is that he (unlike the other more choleric choices people have proposed) doesn't seem the type to force that sort of physical transformation on people? Given how one of the major traits of Ryujin (and a lot of Eastern dragons, honestly) is access to a human form that doesn't retain any of those classically draconic features, forcing those more animalistic/serpentine traits onto someone doesn't feel like it makes sense. The rest of the cost also doesn't really hold up; unlike Western dragons, Eastern dragons (and the dragon god especially) are not generally presented as [i]avaricious[/i] to the point of forcing the collection of treasures in any context. The other part of that is...

Taking really long baths or swimming in a pool? I dunno, seems more like a reward to some people to me.

There's also the dangling thread in the backstory that is the serpent, which is sort of used as an "ooh, look, danger noodle!" sort of reference that is promptly forgotten to tie the character to the school. I assume this is in part a holdover from the Orochi concept from earlier, but still.

That aside, the powerset feels sort of scattered to me. A focus on the ocean and water makes [i]sense[/i], but coupled with the limitations in play that we have for both this character and Rain's own and we end up with a bit of overlap. Immunity to toxins and poisons I'll... Sort of give a pass for now, since he holds fairly notable ties to medicine and healing in mythology. Healing blood I'm not going to say too much on for similar reasons, and talking to sea creatures...

Well, that's the sort of Aquaman ability that we have to force scenarios to have that be useful in at best. Funny, but can be superfluous, so I'm not going to needle at that one too much.

My suggestion is to lean a bit more into some of the mythologies associated with Ryujin if you want to go all-in here and not find a different route forward. For example, Urashima Taro is tangentially related by dint of traveling to Ryugu-jo, which is still his domain, and the myth of Tawara Touda slaying the oomukade also features the Dragon King as a notable side character. Either way, there's a bit of room to draw upon syncretism and more abstract things beyond what's being presented...

But, well, that's just my opinion and not something you need to follow to the letter.

Edit: [@Teyao] (not like the mention will lock in, but still), I don't think I have any qualms about Mr. Narrator here. It's not usurping control, so I have no reason to shoot it down.