The 15th Servant. Since she is the Possession type, she can be considered an incarnated Servant. The possessing Servant is a Caster. True Name: Otachibanahime.
She is Yamato Takeru's wife. The "oto" part of her name used to mean "younger person", but as the language evolved, it came to mean "younger brother", therefore, she more often calls herself Tachibanahime.
She has no direct combat abilities as a Servant and even her main Caster powers are support abilities, so it's hard to imagine her ever winning a regular Holy Grail War.
As Ogasawara Kaya, she is a cheerful, intrusive, and adorable sister who cherishes her older brother. As Ototachibanahime, she is more like a girl single-mindedly devoted to Yamato Takeru.
Their being summoned together is a miracle that will never happen again and both know that. That said, their relationship was already concluded in their lifetime, which might be the main reason why they were able to choose the option silently watching each other from behind.