He's the first character to get jealous, all right - because in the case of most of the romantic/sexual relationships in this series, the primary relationships are established and secure before other lovers are brought in. It's, "I love you, but you love him, but that's okay because I know I'm the center of your life and I love you more." That hasn't been established with MiA and Koichi at the start of this fic - so Koichi sees MiA's relationship with Uruha as a threat to his becoming MiA's "hub." Fortunately, it all worked out for everyone!