Yuka is Kouta's cousin. She is also around 19 years old and planning to attend the same university as Kouta. She moves in with him at the inn after having not seen him in many years. Having had a crush on Kouta since childhood, she is in love with him, and is therefore jealous of Kouta's attention to Nyū, to the point of being very irrational. (Source: Wikipedia)
In the Anime
After the incident of capturing Lucy. It was shown that they lived back again to their inn, living their normal lives.
In the Manga
It was heavily implied that Yuka married Kouta since Kouta's daughter looked exactly like her.