I do like Akane, but mature full figured dragon Onee-San is waaaaay better. Also, he is not her boyfriend. So giving him shit about being with another girl when she's failed to stake her claim on him after all this time is some BS. You want to be the one and only girl, then step up and say it loud and clear.
One of the tropes I dislike the most in these kind of mangas is the jealous girl "friend" who isn't ready to be in a relationship with the boy, but runs every girl off that is, leaving the guy out on some island of perputual single guy status.