Yao

Yao is an eight-tailed kitsune. As a result, she is the second most strongest kitsune next to Tamamo and is her right handed soldier. She is very prideful and sexually sadistic, though not necessarily cruel. Yao also enjoys raping men using her ass.

After Luka defeats the Elder Succubus, Yao appears and attacks, only to be sealed into an eight-tailed fox.

Encyclopedia Entry
"An incredibly powerful monster, Yao is near the top of all monsters in physical and magical abilities. Together with Nanabi, she serves as the right hand of Tamamo.

''Yao enjoys playing with men, and will use all eight of her tails to that end. In addition, she can use her soft giant hand and elastic tongue to delight any man. Incredibly prideful, she doesn't use her vagina to mate with men. When raping a man, she always uses her ass.''

''Though she is sexually sadistic, she is not so cruel as to kill her catch. That said, there's no escape from her if she chooses to play with the man until his natural death."''

Battle Overview
Although stronger than Nanabi in many ways, Yao is not classified as a boss monster and is not potentially threatening.

First, start off with Undine + Serene Demon Sword, then fall back to Sylph + Gnome + Earth Rumbling Decapitation. Alternatively, the latter combo can be replaced with Salamander + Death Sword Chaos Star, however this puts Luka at a severe risk to Mount followed by Gate of Defilement as well as her Eyes of Temptation.

There are three key concerns in this battle: Mount, which Gnome is needed to break free if caught otherwise it leads to Gate of Defilement which ultimately leads to a KO. Eyes of Temptation, which Sylph is needed to block. And Eight Moons, a dangerous step-up from Nanabi's Seven Moon which only requires one turn of charge and hits eight times each more powerful than Nanabi; Gnome and Guard is necessary to evade serious damage.

Succumbing, Yao uses her elastic tongue followed by a blowjob on Luka. If he loses to Gate of Defilement, she will rape him with her asshole, stating she is using her vagina until she lifts up her skirt at the end. Either way, he becomes her sex toy.

Trivia

 * Yao is the only kitsune that keeps Luka as a sex toy rather than mating with him.
 * Yao is the only kitsune that anally rapes Luka.