Koutarou is Rinko's former colleague when they were traveling biologists. He was with Rinko when they found a Kaiju skeleton and watched as Rinko decided to take home an egg, vowing him to secrecy. While he does not approve of how Rinko raised Kuroe, he acts like a protective uncle to her, which causes Arata at one point to mistake him for a boyfriend. Koutarou has since started working for the Japanese government, which has alarmed Rinko, who just wants Kuroe and her newly hatched sibling to be happy.

(Source: Wikipedia)