na-adjective, noun
tsundere; hot-cold personality
Describes a character who is initially cold, aloof, or hostile but gradually reveals a warm, lovestruck side. Common in anime, manga, and otaku culture; now widely understood in Japanese pop culture.
彼女はツンデレで、普段は冷たいけど時々すごく優しくなる。
She's a tsundere; she's usually cold but sometimes becomes incredibly sweet.
あのアニメのヒロインは典型的なツンデレだ。
The heroine of that anime is a classic tsundere.