Satsuki Yatouji is an unsung character of X. But she's an excellent character for the times. Since she is quite into computers, or would that be computers are quite into her? She built the supercomputer, the Beast; which has a serial code of 666. And physically connects herself to the Beast to find out any information she wants or to cause any kind of damage she wants. I suppose if you had to give Satsuki an "element" to control it would be technology. And she is roughly between the ages of 18 and 21 in the X manga and 14 in her side story.
Satsuki is not what I (or most people) would call a people person. She likes to be alone and figures the human race to be inferior and incapable of learning from their mistakes. She's really only come in contact with Yuuto and Kanoe. Yuuto is probably the closest person Satsuki has to a real human friend, and possible love interest. Kanoe, on the other hand, more than half the time has Satsuki doing some kind of research. Kanoe is also very friendly with Satsuki. By friendly, I mean Kanoe often lifts Satsuki's chin or runs her fingers down Satsuki's throat. It's just a little weird.
It is hard to figure out which of the Seven Seals is Satsuki's counterpart. Though I would like to think it is Yuzuriha Nekoi. Since Satsuki has already fought her a couple of times and suceeded in killing her Inuki. Satsuki also mentally tormented the little girl in Volume 13 with her famous question, "Why is it wrong to kill humans?" Recently I have also come across the notion that Satsuki and Yuzuriha are indeed counterparts. Especially seeing how they both have their "pets". Yuzuriha with her Inuki and Satsuki with the Beast. However their relationships with their pets are a little strange. While Yuzuriha's relationship is rather friendly and playful, even protective; Satsuki's relationship with the Beast is one of domination, power and even pain. However it is difficult to tell what both of their fates will be. This is CLAMP, we're talking about here. Please keep in mind these are merely my thoughts on the subject. And most likely should not be taken too seriously, I'm only writing things based on my opinions of the matter.