Akito does not realize this, though. Why would he? For Akito, it's simple enough for him to remain by her side. As long as they stay together, he can convey his feelings for her. In a sense, she, too, shares the same feelings as him. As long as they are together, one day, these walls would naturally crumble down. The things she doesn't know about Akito. His many different sides that other people have learned. She will learn them better. All it takes is patience and time. However, that's something she feels she doesn't have.
If she doesn't complete the mission, she can't stay here. The conditions they gave her even if she stays without completing is far too cruel. Akito will forget about her? She can't allow something like that to happen. Setsura was not confident in winning his heart a second time.