Two years ago, he made a promise with Shiki to attend college together. He discovers a doll at an exhibition his friend asked him to attend and falls in love with its flawless craftsmanship. He is so charmed by it that he seeks out the maker, Aozaki Touko, and then finds her residence.
With his hopes of attending college with Shiki crushed because she falls into a coma, he drops out of college to work under Touko as an assistant investigator. Touko holds his investigative skills with high-regard because of his ability to find her workplace, an abandoned building hidden to all except those who wish to find it. His character design was used as a basis for Tsukihime protagonist Tohno Shiki.
Ryougi Shiki often refers to him as Kokutou, saying that his name reminds her of a French Poet. The poet in question is Jean Cocteau, whose Romanized name is pronounced in a nearly identical manner.
an extended description!