Keita is one of Kotoko's classmates in nursing school. He is very passionate about nursing and can therefore be very fierce if things don't go right. Naoki was jealous because of Keita's and Kotoko's relationship.

Spoiler

Keita liked Kotoko once, but after he realized how much Kotoko and Naoki loved each other, he gave up. Finally, he met the handicapped Akiko during training and fell for her.

Source: Wikipedia