Izumo Tenka
Izumo Tenka is probably one of the best girls to ever be in Mato Seihei no Slave. She is the very skilled Commander of the 6th Unit in the Anti-Demon Corps, and she always steals the show when she is around. Some of the other girls in the series might beat around the bush when it comes to Yuuki, but Tenka does not waste a second. She is refreshingly honest, openly thirsty, and quickly claims him as her own. It is really cool that the main female character knows exactly what she wants.
She looks like she fits every onee-san trope perfectly. Tenka has beautiful long lavender hair and a figure that is so generous that her tight military uniform barely fits her. Her signature peaked cap and thigh-high boots make her look very powerful, which is very different from how sweet and caring she is toward Yuuki. Fans go crazy for her because she does not follow the usual tsundere pattern at all. Instead, she showers her crush with aggressive love and highly anticipated ecchi rewards. She is sure of herself, dangerously flirtatious, and really cares about her team. That perfect mix of a fierce commander on the battlefield and a sneaky romantic partner makes her a top-tier waifu.
Tenka is a treasure trove of fan art and wallpapers. Artists love to draw her in sexy poses that show off her beautiful curves and that famous teasing smile. There are a lot of great background art pieces of her in her sharp Anti-Demon Corps uniform or completely naked for one of her famous reward scenes. Having a Tenka wallpaper on your desktop is pretty much proof that you have great taste in top-tier anime women.