(Manga/anime slang) to become affectionate or act lovestruck—suddenly showing a soft, fawning side (the 'dere' part of tsundere).
彼はツンツンしているが、二人きりになると急にデレる。
He's usually prickly, but he suddenly becomes affectionate when they're alone together.
そのキャラはイベントでファンにデレて、みんなが驚いた。
That character acted affectionate toward fans at the event, and everyone was surprised.
恋人の前だとついデレてしまう自分がいる。
I find myself becoming gushy in front of my partner.