

So I was right; you only get her after defeating Yamata no Orochi (which is plus disk's omote last boss). Her library cost is an extremely low 31, so those low stats can be pretty tweakable.
Her skills and spells:
Ability to Remember Everything Seen Once (1*25):
If Akyuu is in the party, the "View Ally/Enemy Information" menu
will display all enemies' current HP.
Knowledge of Gensokyo Chronicles (1*20):
When the skill holder is in the front,
all frontliners inflict 16% more damage when targetting enemy weakness.
Memories of Gensokyo (1*15)
When the skill holder receives a turn,
remove one negative status from all frontliners.
When both status ailment and debuffs are in place, debuffs will be prioritized.
Power of the Child of Miare (9*4)
If the skill holder's HP becomes 0 while having at least 1 TP,
there is a (SLv*10)% chance to consume 12 TP to fully restore HP.
This effect cannot activate if TP is 0.
Super Incantation (5*10)
When the skill holder uses Concentrate, grant the effect
"next attack's damage will be multiplied by (1.5+SLv*0.6)".
(Holy crap)
Phenomenal Force of Will (1*8)
When the skill holder receives a turn, restore 1 MP.
Protecting Art of Wisdom
MP Cost 12 / Target Single Ally: FIR / Auxillary
Grants the special status "Temporary Invincibility", which negates damage from enemy attacks once.
After "Temporary Invincibility" activates, or 20% of the time when the affected character
receives a turn, remove the effect.
Guarding Art of Wisdom
MP Cost 6 / Target All Frontliners: CLD / Auxillary
Grants the special status "Temporary Invulnerability", which negates status ailments from enemy attacks once.
After "Temporary Invulnerability" activates, or 10% of the time when the affected character
receives a turn, remove the effect.
[Although the spell claims to affect all frontliners, it actually only affects a single target. One of the corridor skills support this as well.]
Corridor skills:
Desire for Greater Wisdom (1*50)
Occasionally "Protecting Art of Wisdom" and "Guarding Art of Wisdom" will affect all frontliners.
Protector of Knowledge (1*75)
"Temporary Invincibility" and "Temporary Invulnerability" will naturally disappear half as often.
True Maiden of Are (1*50)
"Power of the Child of Miare" will occasionally not consume TP when it activates.
Miare's Great Knowledge (5*25)
Mp Cost 11 / Targets all allies / NTR / Auxillary
A support spell that places a "increase next action's damage" boost on all allies.
Tested stuff:
You can view enemy current HP by cancelling out of action menu, then move the cursor to the enemy in question.
Unfortunately, either you can't view enemy stats yet or it's hidden behind some command that I don't know of. The "view enemy stats" part is actually the info text for when you cancel out of action menu, which is translated as "View Ally/Enemy Information" in v1.203 main.
Both of her support spells have 6000 delay. Increasing their level seems to do nothing.
Power of Child of Miare does not reset ATB to 0 when it activates. Also despite the description, it only seems to activate when Akyuu actually has enough TP to pay the cost.
Miare's Great Knowledge grants 34% damage buff on level 1, and 50% on level 5.