To most, Yoshii Yuka appears to be a shy, somewhat-tsundere female who tends to keep to herself at school, but unknown to them, she hides a secret obsession with toying with herself while alone. She becomes sexually involved with Murayama Sawayka after "interrupting" his perverted actions one day at school.
Known by the majority of her classmates and others as a somewhat shy individual with a mild Tsundere personality who tends to keep to herself, Yoshii Yuka goes about her life in a way that she sees fit for herself. However, there is a dark side to Yoshii that no one knows about, and it is that she is really a sex-addicted freak who will go to extreme lengths in order to pleasure herself with toys whenever she is alone back at her house. Day after day, she mangages to suppress her wild, lustful part of her personality while she is at school, but when she accidentally becomes involved with the lowly school nerd Murayama Sawayka after he "unloads his stress" upon her face, she loses all control of her sexual desires and goes into a trance-like state, in which she craves nothing but having sex.
Yoshii Yuka was originally created by Takeda Hiromitsu for use in the Tsundero manga, and it is not fully known as to what exactly inspired her creation. She first appears in Tsundero #1 inside of the Tsundero 01 chapter (Chapter Six).
Being a female character of a Hentai franchise, Yoshii Yuka does not undergo any major changes, outside of opening up to the shell-shocked Murayama Sawayka and allowing for him to become her first true boyfriend.
Out of all of the characters from Tsundero, Yoshi Yuka is the only one featured in media outside of the main Tsundero volume. She has several PVC figures on the market that are produced by Orchid Seed and other figure companies and are rated 18+ (for strong sexual content, nudity, and adult themes). One such example is seen below: