Subaru is an onmyouji, and practises onmyoujutsu. He's the Thirteenth Clan Head, and also the most powerful onmyouji in the family's history. The Sumeragi traditionally work with the government to maintain balance in Japan, and keep track of all other practising onmyouji in the country. The exception is the Sakurazukamori, their opposite, who apparently answers to no one and protects Japan "from the shadows" i.e. doing the dirty work of assassination.
Subaru's job mainly consists of performing exorcisms on spirits that refuse to move on, although sometimes cases become more complicated. Much of Tokyo Babylon focuses on cases he undertakes, although the interest of the story lies less with the mysteries of what might be going on, and more with the way Subaru handles the troubles he encounters. Despite his difficult training and difficult job he remains a fundamentally good and kind person, and exposure to the darker side of humanity through his work affects him deeply. As Hokuto says, he feels the pain of others as though it belongs to him, and never considers his own emotions.
In X it is revealed that Subaru is also a Seal, but since he displays no Seal abilities - kekkai, almost-flight, it can be presumed that either they manifested closer to 1999, or he didn't have those abilities then because he hadn't yet decided on his special person.
