Tsundere

Tsundere is a Japanese term for a character development process that depicts a character with an initially harsh personality, hard on the outside, guarded and reserved with strangers, who gradually reveals a warmer, friendlier side over time. The word joins the ideophones tsun tsun, morosely or standoffish, and dere dere, lovey-dovey or fawning. It was originally used in Japanese bishojo (dating-sim) games and made popular by the visual novel Rumbling Hearts. Comiket organizer Koichi Ichikawa calls Lum of Urusei Yatsura the first tsundere; manga critic Jason Thompson names Madoka Ayukawa of Kimagure Orange Road the root of the archetype.

The arc, per the article: the character is introduced critical of and looking down on the main character, then warms, sometimes to the point of falling in love while finding the feelings hard to admit. That built-in reversal makes the label stock vocabulary on the light novel and visual novel shelves and a ready-made archetype for harem casts.