A tsundere is a character who is initially cold, prickly, or hostile toward someone they are drawn to, and gradually or intermittently reveals a warm, affectionate side. The word blends tsun (aloof) with dere (lovestruck).
The tsundere is built from two sound-words: tsun tsun, aloof and prickly, and dere dere, lovestruck and soft. The character oscillates between them, or travels from one to the other across a story, and the comedy and romance both live in the gap between the public defence and the private feeling.
The archetype codified in the 2000s, in dating games and the light novels and anime that adopted their vocabulary, and it promptly subdivided: the classical tsundere who warms slowly and stays warm, and the modern type who switches by the scene. Signature lines became so codified that they now read as genre quotation.
Like all -dere words, tsundere began as fan and industry shorthand, a way to sort characters by their affection pattern. Used well it is a precise critical term; the label fits characters in any medium, and spotting the pattern in older works, decades before the word existed, is a standard fan pleasure. The pattern long predates its name, and part of the word's success is that it let fans point at a century of prickly sweethearts across world literature and say: that one, exactly that one.
In use"Sharp words in the hallway, a homemade lunch left on the desk by lunch: the tsundere pattern in one move."