[@Zeroth]: Right, form by form then.

Re: Mika:
[list][*]This bit:[quote]Though she’s petite, she’s also filled out rather nicely–and she knows it.[/quote]Could probably be phrased a bit better. Yeah, that's how I'll put that. Could be phrased better.
[*]Personality seems like it clashes with itself. A girl who tends towards teasing while also shouldering the burden of a clan's livelihood on her shoulders doesn't quite add up; if her wish is to raise it back up, you'd sort of expect her façade to reflect that to some extent, right? A personality more in line with that of a gyaru's would reflect similarly, and in this SPECIFIC case where first impressions matter a [i]lot[/i], letting that "true self" bleed through in either direction (at least for starters) doesn't make much sense to me. In my eyes, the character needs to pick a direction and [i]commit[/i] to it. That sorta applies to the whole form, honestly; pick a direction and stick with it instead of going 50/50.
[*]Onmyodo technique names: I humbly request you reconsider how these are named. For example, "Kekkai no Jutsu" literally translates to "Technique of Barriers"　or "Barrier Technique" (or, if you're really stretching it, Barrier Style or something like that). This is very much a 'me' problem in this context, but, like... Yeah. I'll just leave it at that.
As for the actual usage of onmyodo... See, logically onmyoji and all that stuff [i]should[/i] exist in the world, but in my head Attunement had only really started to come into being around the late Heian in Japan—so, like, around the events of the Genpei War, more or less. Onmyodo as a technique would likely be a line of techniques derived from using ether, and so would not require a Contract to use to begin with. Of course, not all humans should be able to use those techniques to begin with; maybe it'd be apt to call it a certain 'type' of ether that can be manipulated without outside interference only in certain individuals? That's probably the best way to explain it.
[*]Backstory... Hm. See, with the knowledge of how onmyodo [i]should[/i] be, the story only half holds up... And, well, not just that, but also sort of doesn't match the personality (or, well, in my view, [i]personalities[/i]) given?
On one hand, we have the personality, which is (in my opinion) a clash of two personality types that don't make sense without context. On the other, we have the actual backstory, which... Is [i]way[/i] more traumatic than I'd expect given the character's personality. You don't just recover from being backstabbed and seeing [i]all your friends and family killed[/i], much less to the point where you'd be able to tease and crack jokes with other people and be so [i]open[/i] with them. You know, after a [i]betrayal[/i] after doing [i]exactly that[/i]?
It just... Doesn't make sense. Then you have the other half of the personality to compare to: a desire to 'rebuild' and 'recover' the prestige of a clan. On the initial read, I assumed this to mean that they were 'in decline', not 'wiped out'. The former would make a lot more sense, especially in matters where pride and public perception are involved, but... That isn't really applicable in this context. You have a backstory that seeks vengeance after a devastating betrayal and a personality that makes it seem like the weight of her family rests on her shoulders AND an open enough personality to be affable with everyone, but... These three cannot all coexist within the same character.
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Re: Kyozan:
[list][*]Uh... A 1000+ year old rakshasa? Let's start putting things into perspective.
First, rakshasa bit. What bugs me more than anything with this section is that they're literally just described as 'oni, but better', which... Feels super off-putting to me? There are probably better ways to address the differences in this context without just going that route.
And besides, it's not as if oni [i]can't[/i] do all the things you say they can't. Oni who are smart, who can use magic, who can use more than just weapons of brute force... They existed and continue to exist in-universe. There's no reason they [i]wouldn't[/i], given the multitude of myths that state they can do just that. Being the traditional (and stereotypical) norm isn't quite right in the context of this world and the way it has developed, honestly speaking.
Now, the age. '1000 years ago' translated into a more apparent year is approximately 1000AD (duh), which places us right around when Heian-era Japan was starting to decline. Now, a thousand years is a [i]damn[/i] long time, obviously, but if we use that and reference a bit later in the form: [quote]Being sealed for hundreds of years by the Aomori Clan[/quote] things get a bit weird. Now, while this [i]does[/i] put us at the start of Attunement within Japan (relatively speaking), relationships between human and youkai would have still been pretty poor for at least a few decades (if not centuries) more. Where am I going with this?
Well, as stated in the OP, a lot of ether's needed for a youkai to really start ramping up in power. That's a lot of kills. Racking up that amount of kills in that timeframe (as you put 1000+ and not, say, 1100+, I'm estimating it to be sub-100 years of chaos) [i]solo[/i] is insane and rather unfeasible unless he [i]really[/i] went ham murdering people... Which, again, paints a much bigger target on his back and probably has him approaching, like, famous named youkai status—[i]doubly[/i] so if the battle left a [i]canyon[/i] in it's wake. If he's [i]older[/i] than 1000, though, even if by a factor of 100-200 years, that [i]definitely[/i] puts him on Big Three level, which... Yeah, no, I'm not allowing that, even if they are depowered to some extent.
[*]Backstory. I get... Like, nothing reasonable here. Why was he sealed? Why does he have affection for the people who sealed him away? We get the destruction of the seal and the temple, but no meaningful [i]reason[/i] behind the actions that he takes afterwards. A throwaway line about a promise or a vague allusion to watching generation upon generation of humans coming and going isn't enough to justify [i]any[/i] of what I just mentioned. I don't have a clue as to why he was as strong as he was, or why he was designated to be hunted, or even why he took any of the actions that he did after popping out of the seal. I can assume, yes, but that assumption alone isn't enough to let me give the okay here. This is also assuming that the battle you allude to at the end of this section did not result in a complete rout, because if it [i]did[/i], there wouldn't be any 'vengeance' motivation in play for Mika.
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That should sum up most of my thoughts on the forms.