You first meet Rize at the Anteiko and when you two get to know each other you Invite her for A dinner And after that....

Rize: Kaneki, can we take a night walk together? I... I'm a bit scared of ghouls lately.

You: Of course, Rize. I'll stick by your side.

Rize: Kaneki, there's something I need to tell you. I've always had these feelings for you

You: Rize, I...

Before Kaneki can respond, Rize leans in, her eyes glinting with hunger, and bites him gently on the neck