Mihoshi used to be a very competent member of the GP but something happened and now she has to wear a ribbon at all times.
from tenchi 101
72. Why does Mihoshi never undo her pony-tail?
Answer:
Because if she does, she will lose her memory.
More details ...
She ties her hair in pony-tail even in the bath. This is because she will not remember anything while she is without it. Why? It cannot be revealed now.
in gxp 2 seina's roommates notice that seina's abilities are like the Kuramitsu's so they tell him to be careful because there are many hobbyists in the galaxy who gather strange abilitied people and turn them into research subjects.
In gxp 11 it is said of mikumo her grandfather "his grand daughter Mihoshi had a past mental illness and was in the middle of receiving treatment for it."
In GXP3 it is revealed by miwase, Mihoshi's former underclassman who had a thing for her, that a spy had infiltrated the ranks of the army devision of the GP where Mihoshi was working at the time and a gunfight ensued and in the battle Mihoshi had her brain blown out of her head. Even with your brain immediately repaired by nanobots, the brain cant normally recover from the shock and psychological trauma of being blown up. it is stated by miwase when he notices that seiryo recovers abnormally quick from trauma
later on in the chapter it is revealed that Mihoshi had her brain repaired using a biometal that when deformed regains it's memorised shape when exposed to heat and that pirates had used this to effectively replace students brains and control them as spies using nano machine implants it must have been a pretty unique procedure at the time because it specifically is reference as being just like what was done when mihoshi had her brains blown out. it's also said this is what cause her to leave the army division of the gp and become a field officer.
How is that possible that mihoshi got shot if Mihoshi's ability is to skew probability improbably in her favor? While not much on it is ever stated other than in these specific spots, it would make lots of sense if it was a human hunter that had sent in the spy because they would specialise in overcoming her probability bias.
The picture from ova 3 shows her brain hollow in the center and her life data as the background with her heart also for some reason hollowed out and her arms are segmented as well...
Misao was worried about a repeat of her being in this state, so he went and took the chobimaru to tenchi so it's also likely that it didn't just end at her brain being blown out but rather they successfully captured her body after words and ran experiments. unless this is just a picture of her while doctors were trying to figure out what to do with her body after the shootout.
Its also entirely possible that this is another incident entirely... but unlikely.