She is Rin's mother and died because of an illness when Rin was very young.

3rd grade teacher Aoki Daisuke didn't expect the first class he ever taught to be one of the toughest obstacles of his life. After getting off on the wrong foot with the entire class, a moment of kind-heartedness instantly convinces one of his students, Rin Kokonoe, to make the ill-fated Aoki-sensei her lover. But what exactly are Rin's intentions for wanting to actively seduce Aoki-sensei, and will Aoki-sensei be able to help her to deal with them?

Birth name: Tomoko (智子) Yoshikawa (吉川)

The story of Kodomo no Jikan is centered around 23-year-old Aoki Daisuke, who has just landed his first teaching job as a grade school teacher at Futatsubashi Elementary School. He is placed in charge of Class 3-1, where one of his students, a mischievously precocious Rin Kokonoe, develops a crush on him and goes so far as to proclaim herself Aoki's girlfriend. She aggressively pursues her efforts to be with her teacher despite the fact that he will lose his job if she gets too close, a situation further complicated by the often complex, intertwining relationships existing between them and their respective friends, families, and peers.<br><br>
(Source: Wikipedia)

Aki Kokonoe