Tsundere describes a character who acts dismissive and only gradually shows affection. The gruffness isn't a character trait, but a shield.
This term also comes from Japan and refers to the switch between two attitudes: first the rejection, then the awkward care. In Western romance vocabulary, it's close to Grumpy × Sunshine, but more narrowly defined — with tsundere, the harshness is deliberately played, and both parties know it.
The appeal lies in the mismatch between word and deed. She tells you to get lost while clearing a chair for you. He claims he doesn't care and still shows up at your door at two-thirty.
For interactive stories, this is a rewarding type because he responds to your persistence. If you stay calm, the facade crumbles faster than if you push back.
At Everglow you do not just read it. You are the protagonist, and every chapter ends with your move.