Ms. Mikami taking a hit from Yukito who was trying to kill Mei. This in turn caused Kouichito punch Yukito. On the surface it seems like he was simply defending his teacher and his friend. However when we find out later on that Ms. Mikami is his aunt Reiko, it's a double whammy of heartwarming. The first is that the Reiko side of Mikami only knew Mei on screen in the beach episode, whilst she knew but didn't acknowledge her during Mei's term as the Non-Existent student. Yet because she is her nephew's friend, she defends her regardless. The second whammy is that it justifies Kouichi flying off the handle and punching Yukito, because he was not only trying to kill Mei, but nearly killed his aunt in the process.
The entire montage of Episode 6 becomes heartwarming as Mei and Kouichi spend time together.