Aika Kiryuu

Aika is a second-year student who is in the same class as Asia and the Perverted Trio. She wears spectacles and like Motohama, she has the ability to calculate a male's "manhood" size just by looking. A good friend of Asia, she likes to tease the latter about her crush on Issei. One of her most well-known traits is her habit of giving lewd advice to Asia, such as telling her about "skinship" and encouraging her to try it with Issei in the bathroom and warning her not to dilly-dally in making a move on Issei before the other girls could get him. Despite Issei and his friend's perversion towards girls, Aika appears to be the one exception. This is most likely because she acts more like them, rather than a typical girl. Aika eventually finds out about Issei being a devil. (Source: High school DxD Wikia)