Setsuna is a disciple of the same school of demon-fighting sword arts as Motoko of Love Hina. This means that in addition to superb swordfighting skills, she can use Eastern spell talismans/charms and knows how to channel Ki for unarmed combat. Konoka was close friends with Setsuna when they were children but Setsuna couldn't save her from almost drowning. With this incident in mind, Setsuna trained hard to become a competent (read: aloof) bodyguard. She always kept her distance and was generally unsociable. After a very large incident during the school trip to Kyoto (which the anime largely skips to the conclusion), Setsuna starts to shed the icy air (the alternate anime skips straight to this point of character development) and acts friendly with Konoka. Actually, it's very very easy to read shoujo-ai between these two (Setsuna's blushes are one sign), especially since Konoka teases/molests her as if she was a shy boyfriend.
Incidentally, her species is not human. Her true form adds grace to her but before her friends corrected her, she was ashamed of it and felt she had to disappear from everyone if she ever revealed it.