Vermilion Akiha

Akiha in a state close to Crimson Red Vermilion.
Through sheer willpower alone, she is able to control her rampage and avoid fully becoming a Crimson Red Vermilion.
Or rather, for Akiha this form is actually what she looks like when she's conserving her strength, and her normal form is the result of her actively using her power to suppress her oni blood. It's quite similar to the situation with Arcueid and her vampiric impulse.
Properly speaking, becoming a Crimson Red Vermilion means completely losing your ability to reason, but it seems even in this state Akiha still has some sanity left. This, too, is probably by virtue of her high aptitude for it.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary