Caster is a beautiful, young fox woman with long pink hair tied into pigtails and yellow eyes with bright orange fox ears. She wears bright blue Japanese stylish erotic dress with two separate long blue sleeves, indigo long socks with black geta, and light orange fox tail.

Caster is sometimes comic relief, being a goofy, silly, and cheerful young fox woman. She sometimes take the battle and the situation by each week of the game seriously.

Her real name is Tamamo-no-Mae.

A beautiful woman who served Emperor Toba. She was said to be the disguised figure of the white-faced, nine-tailed fox. Her childhood name was Misukume (藻女). Working in the palace since 18 years-old, she later became a court lady of Emperor Toba under the name of Tamamo-no-Mae. She was said to have gradually gained the favor of Emperor Toba with her beauty and extensive knowledge. She was said to have the looks of a twenty-year-old.

In Fate/Extra, Tamamo-no-Mae is a soul that branched off from Amaterasu and her true identity is not that of a nine-tailed fox, but rather is treated as one of the facets of said goddess. Also, its a big mistake to categorize her as an Heroic Spirit. Because Tamamo-no-Mae turned into a Heroic Spirit in order to realize her wish of "becoming a good wife," she cannot manifest her original power. In the case that she is revived as an evil spirit, she will undergo a transformation into a large monster that can defeat even 100 heroes. It is not just for show that she is called one of Japan's Three Great Monsters. There are many theories, but later she was driven away from the palace when her identity was exposed by a Onmyouji who investigated and identified her as the cause to why Emperor Toba had fallen seriously ill. After leaving the palace, she built a bad reputation in the fields of Nasu and had to repel a subjugation army of 80,000 men dispatched under the Emperor's order.
(Source: TYPE-MOON Wiki)