T91 (Type 91) is a taiwanese assault rifle and a personified anime character first in the manga by Kitsune Tennouji called Upotte!! which had an anime adaptation and later in the chinese mobile gacha game Girls Frontline (to be trained eventually), despite the both looking totally different to each other in their respective IPs.
She's always seen getting along with Steyr AUG (which I'll also eventually train due to loving her design and dual personalities!) to assist/support her since presumably they're BFF I guess (you can read the rest about her here).
Here's her trigger tag and tags for her Seishou Academy uniform (like almost all the other personified gun girls wear in the manga/anime):
t91, school uniform, white serafuku, blue sailor collar, red bow, short sleeves, green skirt, pleated skirt, kneehighs, white legwear, sneakers, pink footwear