Satsuki Yatōji (八頭司 颯姫Yatōji Satsuki) is a complex and ingenious young woman with utter contempt for humans.
She has a love for the digital world instead.
At a young age she developed the uncanny ability to interact with computers through cables inserted into her skin, and to hack into any technological system. This led to her father sending her to a lab for further study; she amazed the scientists by mastering the Sephirot at the age of 14.
She rebelled against the boredom she felt and in the anime TV series, through her "friends" in the computer world, arranged for the death of her father (in a road traffic accident), along with anyone else who stood in her way. While escaping from the lab she encountered Kanoe and joined her cause, having learned from the digital world that she was one of the Dragons of Earth; later Yūto forcibly convinced the scientists to never bother her again.
Satsuki is an extreme rationalist, and attacks Yuzuriha when the latter cannot explain why killing humans is more wrong than the killing of the natural world. She spends much of her time wired into a massive supercomputer named the Beast (provided for her by Kanoe), through which she can physically control cables across Tokyo, using them for reconnaissance and offensive purposes. She develops feelings for Yūto, one of the few people who managed to befriend her, and who in fact gave her the answer that Yuzuriha couldn't.
She kills the people in the lab (who are implied to be Freemasons), but it is unknown if complications arose from this or when and where she met Kanoe.
In Tsubasa, Satsuki is one of the fighters of the City Hall faction. She studies medicine and initially receives Syaoran's group with suspicion; however she's notably less apathetic than her X version, her personality more bordering on seriousness.