The girl is cold, blunt, or even aggressive toward the protagonist. This is often a defense mechanism used to hide vulnerability or social insecurity.
Expect grand verbal declarations to be rare and thus explosive when they happen. A final-relationship tsundere might say "I love you" only twice a year—once on a significant anniversary, and once during a moment of extreme vulnerability. But she will show it constantly: keeping his favorite snack stocked, learning to play his favorite video game to spend time with him, or staying up late to help him with a project, all while grumbling about how "troublesome" he is. Lovely Sex with Tsundere Girl -Final- -Completed-