She has the power to summon eight hundred mice, but we have not yet seen it because (unlike the power to summon, say, eight hundred centipedes) it is utterly useless.

Um, no idea. I like that she and Hakkai both have the character "eight" in their names, since they get along so well. (From the Japanese "ya/ttsu" "eight (physical objects)" in her case, I assume, rather than "ha/chi" ("(number) eight.") Perhaps eight has some symbolism that applies to both of them?