Ok, this is quite a big work.
Serene Yokai:
-Does Domain triggers if the Serene Youkai themselves approaches an enemy after activating? If yes, then that sounds really abuseable as you can basically step forth and back for multiple triggers.
-What's the activation action for Selfless?
-"Hostile mystical effects" isn't very clear. Maybe Su and spell/SLA attacks would be more precise?
-How does Reflect Torment works with area attacks? Say somebody throws a fireball and they make the save while having some nearby allies, does the fireball affects the allies?
-Liberate Magic's CL cap sounds kinda too low, it will only go up to level 10. Didn't you mean spell level instead of caster level?
-For the feats, may be better to clarify that other levels just add to effective Serene Yokai level for abilities that care about that, otherwise it may be read that other levels will grant class features faster than normal. Also Monster 13/Serene Youkai 7 would still be limited to Liberate Magic only up to 13 CL which will struggle against magic enemies of the same level.
Tyrant Youkai:
-Checked may want a clause that it affects undeads and constructs despite being a Fort save.
-Devastating Strike/Cataclysmic may want for the initial attack(s) to be limited to basic ones.
-Just Uncanny Dodge for 3rd level feels kinda lackluster.
-Improved Uncanny Dodge at 6th level alone isn't that hot either.
-Greater Tyrant Yokai appears to be lacking an "additional" there.
-Same comments about "effective X level" stacking clarification than above, assume the same for the other classes.
Coveteous Youkai:
-Pokémon woot!
-Sooth needs clarification about what counts as "mystic" too.
-For Slime Stasis, if you put your slime spirit from afar in their Pokéball, does it fall where they were or comes to your hand? Also when you pull out one out of stasis, where do they appear?
-Sealed Slime sounds a bit too abuseable. Probably better if it was just 1/day and such a slime only lasts 24 hours or something.
-For Rejuvenate , just in case may want to add a clause it doesn't help with recovering time-limited abilities such as daily spells and whatnot.
-Any limitation for spamming Recall Slime until it works besides the time?
-Really like this one!
Progenitor Youkai:
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Spiders are not insects!Vicious Youkai:
-All levels of almost pure melee offensive sounds a bit too much. Could use a bit of fast healing or something in there. Also Chase lacking units for the extra movement speed.
Ravenous Yokai:
-At proficiencies mention of Serene Youkai.
-First level only a boosted bite with crappy skills doesn't feel very good. Deep Stomach could probably be moved here while con to bite attack gets moved to second.
-Eat Big is kinda obsolete by itself at this level since freedom of movement and teleportation start to be pretty common, Improved Grab by itself isn't gonna do much.
-Bulky should clarify that it works like Powerful Build that stacks with actual Powerful Build, otherwise that raises a lot of questions such as space filled and reach and whatnot.
-Tasty Treat probably could use to be extended to protecting from all the nasty anti-melee effects out there.
-When expelling a devoured opponent, where do they end and are they prone or not?
-Must say his one feels quite thematic overall with the whole dimensional stomaches aspect, plenty of possibilities all around that aren't exactly common in D&D. Still could probably use 4 skill points per level base and good Will saves.
Mirage Yokai:
-Proficiencies seems to have some leftover text from Tyrant Youkai.
-Where do Mirage Warriors start?
-"Supernatural abilities merely produce Figments" isn't very clear what that actually does.
Overall I like where this is going, but need some polishing in several parts.