As usual, standard disclaimer that what I write below is my opinion, it's ultimately Elcura and Fawkes' call on what to heed and what to dismiss as too technical.
Hmm... Youmu animations... I note that for both attack animations and the guard animation, Youmu starts with one hand holding the sword somewhat diagonally outwards, then transitioning to one where she holds the sword upright with both hands, before doing the rest of her motion. That feels unnatural - a serious warrior wouldn't be relaxed on the battlefield, especially an Eastern sword martial artist of Youmu's level. I'd say have her initial pose be that "sword upright in both hands" stance. I'm sure those better versed at the sword arts could have a long argument on which of the possible neutral stances is best suited to Youmu but this one looks correct and feels right. In fact, given you've drawn that stance with her rear leg bent and her forward leg straight, it gives the impression of Youmu favouring defence over offense with such a neutral stance but I'm getting too technical now, it's fine as it is.
For the physical attack animation, is there a reason why Youmu lets go of one hand after she slashes? Or rather, it's natural if she has to move her sword that far back after the attack, but why should she swing that far back after the attack? The rest of the animation from "sword upright" to "swing sword" look fine so it's just the final frame that can be edited so the sword isn't too far away - it's unnecessary movement (doubly so as she has to bring the sword back that far to attack again in a real fight) and meaningless movement is frowned on in martial arts.
Both the magical and physical attack animations could do with a little extra speed in the actual attack. Currently it feels like Youmu is practising a new move in a dojo, not fighting in actual combat.
Guard animation... I've discussed with Fawkes on where this animation will be used. It'd be on the "no damage is taken from the enemy attack" event of combat. There is no difference between "Dodged to avoid damage" and "Parried to avoid damage". In which case, wouldn't it be more natural to graze in Touhou than to parry? Having said that, there's nothing to say some characters can't block instead of dodging.
If you choose to replace the Guard animation with a Dodge animation, then not much movement is needed - Youmu just needs to move/lean slightly backwards (either into the paper or backwards in the 2D plane we see her in) to dodge, optionally with a half-step. Again, minimal necessary movements of martial arts.
If you choose to keep the Guard animation... you'll need to discuss with Fawkes on what sorts of attacks he has in mind for the battle animations. What you have there (if you drop the initial pose and go straight from "sword upright" to "block") is a motion and pose for Youmu to block a long, continuous attack or struggle - e.g. a beam or a long stream of bullets. It's inappropriate for a single, focussed and instantaneous attack because she's directly opposing the force of the enemy attack as opposed to the more efficient method of slightly re-directing the force. Japanese swords were shaped the way they are because there wasn't much good steel around - they're forged with emphasis on efficient technique, not on brute strength as European swords were made.
So if Fawkes had the "instantaneous attacks" predominantly in mind for combat animations, then it's best to have a Parry animation instead of a Guard animation, where a single swing of the sword (probably diagonally downwards from her point of view) is more appropriate.
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Now, Asa concept... It's good, I like it. I was pondering where she got such elaborate fashion from, but Touhou has never ever explained where characters get their sometimes unconventionally designed clothes so nothing wrong with that. Heck, her origin is the human village and outside people do arrive there every so often, so there's plenty of opportunity.
Is there any special meaning to the three curves we can see on her left sleeve? It's a nice extra detail to avoid having her upper garment (away from the centre where she has buttons and the brooch) be devoid of features in contrast to her exquisite skirt.
