He snaps. His demeanor goes cold, raging cold. He seems to be caught in his anger. His slightly unfounded anger.
The normally kind, softhearted, and pushover Haru turns into Black Haru, an angry, raging bull. He can't help becoming someone else, a darker version of himself.
But he's still Haru to her. He'd always just be Haru. He'd always be someone she could easily talk to about anything, and he would always be a caring listener.
He never became Black Haru when he was just around her, because they understood each other. They understand each other's insecurities, pride, and coldness.