Sometimes Kurisu wonders if Okabe really loves her.
It's not really that she doubts that Okabe Rintarou loves Makise Kurisu. Because she knows he does, so much that it even transcends various world lines. And maybe that's the problem.
Because she is always so unsure – what if she's only the shadow of another Kurisu he loved but couldn't be with? What if he does love her, in a way, but loves another Kurisu more? She wants to be angry and jealous, but it's hard when she knows that her only and possibly mightiest enemy might just be herself from another world line.
He fell in love with her, yes. But that was, too, in another world line. With another Kurisu that surely also loved – loves him. Because she can tell, there is no world line in which she wouldn't love Okabe Rintarou. And childish as it may be, she's jealous of the Kurisu that received Okabe's love first.
It gets really confusing with the memories from other timelines she sometimes sees in her dreams. Memories that are hers, in a way, and yet so much not hers that it hurts. Even her memory of her first kiss with him belongs to someone else. Someone who is her and yet not.
Maybe she feels so much this way because she barely even remembers through dreams of another her the time in which he fell in love with her. The time in which they fell in love for the first time. She doesn't remember why he loves her as much as he does and the scientist in her tells her that that was another her, someone else. Someone she isn't.
Okabe always notices when she's like that. He explains to her, again and again, that it isn't like that. He fell in love with her because she is Kurisu, not because she is a particular Kurisu from a special world line. It's her that he loves, simply because she is Makise Kurisu and that it isn't as if he fell in love with someone who wasn't her. He fell in love with the same person, only with three weeks experience of what she is like and how very warm and kind and lonely she can be. It's all her, everything, every memory, even if she can't remember wholly.
And she believes him when he says it, because it's times like those, when he is sincere for once, that she knows he means it. The times when he isn't hiding beneath his facade of 'Hououin Kyouma' are the times he's the most honest and she knows that. But sometimes she wonders. Because she is a scientist and she doubts everything.
Even his love for her.