Her full name is Sun Otonashi, she's a Siren, she used to go to Youkai Academy when Ginei was actually a bully kinda guy. Since she's a Siren, she doesn't talk, instead she uses a notebook to talk. Here's something of her from the Rosario Wiki.
"Sun Otonashi (&38899;&28961;,Otonashi Sun) also known as Sun-sempai, is a graduate of the school and the former Newspaper Club leader. She rarely speaks and carries around a sketch book to write on instead of talking. She was the one who taught Ginei Morioka how to use a camera and stopped him from fighting everyone in his first year. She lives with Marin who found her after she had nowhere to go.
She has blue eyes and blue hair. She ties some of her hair back with a ribbon tide onto it. She is older than she looks (Kurumu and the others said this when they found out Sun had graduated). She wears a poka-dot dress (up to her knees) with a big ribbon on her chest and a long-sleeved clothing with a thin ribbon around the sleeve. In her true form, she has a pair of angle wings.
She is a Siren who has a perfect song of protection. However, her true power lies in her ultrasonic offensive song which slowly breaks apart her opponent internally. Gin has remarked that Sun's sketch book is actually her power limiter, and that she is called the "strongest of the generations" in Youkai Academy. She is shown admiring Tsukune."
Here's a couple images of her.
Sun Otonashi B&W. Colored Sun Otonashi. The colored image is from a Manga Cover, just with Ginei cut out XD.
Nisa and Pram........My Strongest Mage and Fist fighter yet and only getting stronger....FLAT POWER! XD *is frozen by Pram then busted into pieces by Nisa*.
Current Obsession: Disgaea 4.......And Hii-chan X3.