NTR: My younger brother’s girlfriend

Yuna arrives one day as his little brother's girlfriend, and that's all it takes for the protagonist's uglier instincts to wake up. The setup is blunt about what it wants: proximity is the opportunity, and he means to use it.

From there it's a straightforward corruption plot. He watches her, finds the cracks in her loyalty, and starts closing the distance between them while his brother stays oblivious in the next room.

Play revolves around pushing that seduction forward step by step, mixing teasing and light blackmail with groping and other advances until her resistance gives out. Scenes are told through 2D stills rather than animation, so the tension sits in the dialogue and the slow escalation rather than movement on screen.