Setsuna is a Shinmeiryu swordswoman from Kyoto who was taken in by Eishun Konoe, Konoka's father and the leader of the Kyoto Mage Association. She is childhood friends with Konoka and her protector.
Setsuna spends the beginning of the series being relatively cold to Konoka, wishing to protect her from the shadows due to not forgiving herself for being unable to save Konoka when she almost drowned when they were kids. She does eventually open up to Konoka after the Kyoto arc and begins calling her "Oujo-sama" and "Kono-chan".
Setsuna's pactio, Sica Shishikushiro is a wakizashi which she can summon and multiply at will to strike targets like missiles. Due to her training she's also skilled in anti-demon arts and is the one who taught Negi how to create shikigami from paper.
Setsuna is a hanyou like Koutaro, in particular a crow. She was expelled from her people because white wings are considered a bad omen, and it was implied by Evangeline that she is an albino using hair dye and contact lenses to appear human.
A running gag within the series is that Setsuna and Konoka appear to have romantic feelings for one another, yet Setsuna will deny it whenever she and Konoka are stumbled upon in romantic situations or asked about her preference.