Suzaku and Nunnally
The two met when they were young at the Kururugi Shrine Lelouch and Nunnally were shipped off to. Their meeting could not be called a meeting. It consisted of Suzaku beating Lelouch, and Nunnally, who pleaded him to stop.
Their first reaction towards one another was fear for Nunnally, and most likely sympathy for Suzaku. Nunnally was scared of the boy who could easily bash her and her brother, and Suzaku, probably had never met a disabled person.
But despite a very rocky first meeting, the two became friends within the first month Nunnally stayed there. When Nunnally was lost somewhere in the huge area of Kururugi's shrine, Suzaku decided to look for her. He didn't know why, he just did because he wanted to help her. An arrogant and proud boy who hated Britannians, cared for a weak and fragile Britannian princess.
Suzaku was the one who found the lost Nunnally. When Lelouch came and found them after, they were laughing. Suzaku was the first to make Nunnally laugh after her mother's death. Suzaku managed to make Nunnally feel safe and happy, after being lost and desolate before he came. This is where the friendship between Nunnally and Suzaku began, which is the bond that led Lelouch to become closer to Suzaku as well.
The rest of the time at Kururugi's Shrine as filled with fun and happiness. But as pure hearted as Nunnally was, she did become a little jealous of Suzaku. As Suzaku and Lelouch grew closer, they did many things together, some of which they could not bring Nunnally because of her disability. Perhaps she felt lonely without either of them, or she resented her disability for not being able to go with Lelouch and Suzaku.
Years later, she attends Ashford College. Suzaku enrolls in Ashford Academy, and Lelouch wants to surprise Nunnally by reuniting the two. Nunnally waits for her surprise, and Suzaku puts his hand in hers. She is shocked; she immediately realizes who it is, and is brought to tears, meeting Suzaku again and knowing that he is safe.
During the course of Code Geass, Suzaku's meetings with Nunnally are rare; he is always busy with military work. But in the few times where he visits Nunnally, they are always happy, laughing together and making jokes. Suzaku visits Nunnally as much as he can, and Nunnally always rejoices at the ocassions when they can be together.
Suzaku and Nunnally obviously care for each other, but Nunnally seems to show feelings a little stronger than friendship. Whether she has a simple school-girl crush on him, or genuinely loves him romantically is not known. In Stage 21, she sadly and regretfully tell to Lelouch that Suzaku and Euphemia really suit one another. She felt a tinge of jealousy.
Nonetheless, Suzaku cares for Nunnally to the state that he would risk his life for her, and I believe Nunnally would risk her life for him too.
