noun
cool on the outside, lovestruck on the inside
Manga/anime slang for a character type who acts cool, aloof, or indifferent but is secretly lovestruck or affectionate. From クール (cool) + デレデレ (lovestruck).
あのクーデレキャラは普段は冷たいけど、たまに見せる笑顔がたまらない。
That kuudere character is usually cold, but the occasional smile they show is irresistible.
彼女はクーデレで、好きな人の前だと急にデレる。
She's a kuudere; she suddenly becomes lovestruck in front of the person she likes.