When Kazuki first meets Yayoi, and like a true gentelman (or prince as Yayoi put it) rescues her without any thought for his own safety.
The very next episode shows him spending all of his money on medicine for her.
One time D accidentally threw Mitsuki's diary away. Then she took her Giant Robot to the nearest junkyard to try and find it.
Pretty much anytime D is around Kazuki.
The ending is the biggest when you really think about it, it's Mitsuki's conscious choice which created the third reality where everyone, including her alternate reality counterpart. Could all live together in peace. She certainly didn't have to do it, but she made one of the most selfless decisions in the entire series. It's also another pleasant contrast between her and Ayuko.
That and the discovery that Zinv survived the self destruct in episode 14. Admit it, you cheered too.