How do you get rid of toxic relationships?

I'm supposedly dating someone right now, but he's eccentric and refuses to give me any information about himself—not his phone number, his picture, or anything else. He's supposedly with someone else, and I'm just being used as an excuse to cheat on his partner with me. He tells me he wants to be with me, but he refuses to define the relationship clearly, saying we're just friends. I want to get out of this to focus on my future and my studies, but I can't seem to end the relationship.