You can build Youmu tanky (HP Youmu) enough to be in the first slot. You pretty need to just use regular attacks, since not doing so will bring her MP down to below max breaking her ability to gain stats.
It also means any time you use a skill, it takes 2 turns of concentration to get back all your MP assuming you maxed that concentration skill, which is a no go for me.
If her MP gimmick is buffed to say either 12 extra MP regen or 4 MP regen when she takes a turn works, then, she can actually use the high MP attacks regularly. Even if the formula so so, at least she'll be able to use those attacks and gain buffs from her Meikyo Shisui.
With this, she will be a decent character, especially with her new skill set. But without any changes, I'm not sure if even her new skill set will save her from being mediocre.
Huh? But... if you're using regular attacks, her damage is going to be abysmal; not even close to being worth activating Meikyo Shisui (it's nice, it's not god. Monk+Strategist puts you at 100% forever which is sweet, but if you're normal attacking what's the point of having those buffs?). You could sub warrior and have someone cast Magic Circuit on her to keep Meikyo Shisui going, and the damage+delay would be okay, but I still don't really see it being worth your time when you could have someone much more useful using up that slot instead. Maybe postgame with her eternal corridor and the Blood Sword for HP drain on normal attacks that would work, actually, but Remilia and Iku would be stiff competition for this job with their own endgame stuff and far stronger normal attack bases to get through DEF/MND with, plus they wouldn't need magic circuit to make their passives functional.
And yeah, if you're willing to Concentrate, her mp really isn't an issue; only Wind of Karma etc costs more than 10 and if you really want to use it, you can use a main equip. If you're fighting a single target boss that isn't weak to WND (a pretty uncommon boss weakness from what I remember...) she'll likely do about the same or more damage with Present Life Slash to be honest, with it's lower def influence. But at that point, you're not a whole lot worse off just using warrior normals. And... hey, wait a second! If you're concentrating every other turn, you're instantly doing half the damage most other characters can pull off, because you're wasting half of your turns doing the same or less damage than they would. zzzzz.
This is why Youmu is bad. If you have to concentrate every other turn, the fact she does the same or less damage as everyone else suddenly turns into doing effectively half their damage. But yeah in end-postgame she'll have a huge damage up to make up for that... albiet it's gimmicky so she'll still be meh, but, vastly more usable. (Plus in postgame you can sorta fix her MND and make her tankier. Or just tweak her HP harder for regen, but.)