He didn't know when he started to like her, or if he just always had.
She was a little weird at first, and he honestly thought she had some weird eating disorder since he never saw her eat. But then he found her stash of protein bars (which he ate a few of) and he worried a little less.
Of course she was pretty, more than pretty actually, and Levi tried to flirt with her. Not that it ever worked, because when he'd try she would be slightly meaner to him.
But somewhere between his hopeless attempts at flirting and driving her to rescue her twin (who apparently didn't need saving), he started to notice the little things about her. He loved the way words rolled from her tongue effortlessly, and that she had Simon Snow plastered everywhere. He loved her glasses (even though they hid her beautiful eyes), and he liked her so much that he didn't know what to do with himself anymore.
So when he showed up at her door looking for Reagan to help him study, he didn't imagine Cather to help him. He didn't imagine that he would sit on her bed and she would read to him until her voice was raw, didn't imagine pulling her against him as she read.
Never did he imagine Cather kissing him, and it feeling so wonderful.
So, no, he didn't know when he started to like her, but he certainly knew that he did. He didn't care about the details, he cared about her.
He cared too much too fast, but he liked her. He liked her so much it hurt.
So he smiled as she kissed him and drifted to sleep just like that.