Yatouji Satsuki is a young student, and technically a genius. She has built her own supercomputer, The Beast, and would rather spend her time with that and the members of the Chi no Ryu than with people her own age.
Satsuki didn't appear very much in the anime. There was the occasional glimpse of her and The Beast, but that was about it. It was rather unremarkable when she was killed, and I don't remember it very well (I think it was either Fuma, or The Beast, that killed her).
She appears to have a more prominent part in the manga, as Kanoe's aide almost. I don't know if she lives with Kanoe or not, and if she doesn't I'm quite unsure why she lurks in her bedroom... She appears to have these special screens where she can remotely interface with The Beast, which appears rather handy ^^;, and uses one when Kanoe qustions her about Nataku. When she goes to find out about Kotori, however, she uses the whole machine. She teases Kotori a little, and The Beast punishes her for it... Kanoe says at one point, "Passwords, viruses, firewalls - all mean nothing to Satsuki...NOTHING! She is without limits--because the computer is in love with her!"
Her computer adores her, and she can do almost anything. She is Kanoe's 'hacker' almost, and can search any database she wishes. Yet at school she stays on her own, because the prefers the company of her computer to any human.
X Japan song:
'Dahlia', because it has almost a cyberpunk quality to it that seems to fit Satsuki well.