little sister
Tsubasa was one of the first people who recognized that Arima had a deeper, darker side. She didn't understand it, but because she herself was used to loneliness and being let down by those she loved, she could identify with him. Because he was kind to her and spoiled her somewhat, she fell in love with him. Despite this, Arima couldn't see her as more than a little sister and practically ignored (though by accident) all of her attempts to confess her feelings to him. And if that weren't bad enough, he fell in love with and started dating Miyazawa while Tsubasa was stuck in the hospital!She couldn't deal with this and selfishly tormented Miyazawa in an attempt to get the two to break up, but what ultimately happened was that she became attached to Miyazawa instead and later fell in love with Kazuma, her step-brother. Tsubasa was a prime example of the fact that just because a person could relate to Arima, it didn't mean that you had a real chance of getting to know him. On the contrary, he would always keep you at a distance.
The most interesting thing to note about the earliest stages when Tsubasa first appears is that her appearance serves to show how utterly obsessed Arima is with Miyazawa, even from the beginning of their relationship. Though he liked Tsubasa and was friends with her, when she lied to him, saying that Yukino had hit her or shoved her, Arima didn't believe her for a second. He simply looked at Miyazawa's eyes for a moment and he knew that Miyazawa was innocent. When he saw Tsubasa later in the hall, he callously asked her to be nice to Miyazawa instead of even attempting conversation.
After Tsubasa began to get over Arima and become friends with Yukino, she started purposely clinging to Yukino to make Arima jealous, showing that she understood him fairly well, for he did indeed get pretty angry. Their friendship was pretty much over at that point, though, because Arima didn't talk to her much after Yukino entered his life.