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Overview
Yakui-san is a very popular if not most popular Nijiura Maid in the west. Her name comes from "Yakuji" (やくじ, "Medicine"). She is addicted to pharmaceutical drugs.
She was kicked out of her parents' house because she was messing with the products of her family's drugstore. Her knowledge of medicines is top-notch, ranging from happy powder to drugs undergoing clinical trials. She has access to both legitimate and illegitimate drugs, and is also able to prepare her own original preparations.
In the early days, she was recommended by Yuki Kotonomiya to be the rightful successor, but she was opposed by the other teachers of Megaku and disappeared.
Appearance
Yakui-san is short because of her drug abuse. She has a purple bob-cut and maid outfit (sometimes the outfit is blue) with purple and white stripes stockings. She is often pictured drooling or vomiting with a crazed look in her eyes.
Once a month, she is tied up and de-medicated to make her more mature. The handcuffs on her arms are a remnant of that time.
Personality
When off her meds, she's rather sensible, and when she's on her game, she's funky and capable, but if she overdoses even a little, she's peaky and useless. When she's on her game, Yakui-san can see things she can't see and hear things he can't hear. She draws a line in the sand when it comes to changing people's minds with drugs.
Because of her drug abuse she is a part of the Inappropriate Maids along with Shitai-san and Dorei-san.