Sougen was a member an organization motoring Mixed-Blood individuals. He was friends with Kiri Nanaya. He later retired and became "some sort of shady back-alley doctor" with his Jinan Clinic. He acted as the exclusive doctor of the Tohno family. Since Shiki Tohno came into the family, he acted as Shiki's primary care physician. He used eastern medicine to treat Shiki's anemia, knowing it's true nature, but it was never something he could cure. He also became acquainted with Miyako Arima due to her following Shiki to his appointments while Shiki lived in the Arima household. He eventually began teaching Miyako basic forms of Bajiquan.
Sougen is referred to as a "mad doctor" by Shiki. He is foul mouthed, but also possesses a strong sense of duty.
 Tsukihime Dokuhon PlusPeriod - TSUKIHIME Dictionary: Jinan Sougen [Person's name], p.182
Sougen Jinan [Person's name]
As the Tohno's exclusive doctor, he is Shiki's primary care physician.
It seems that he used to be a member of an organization that monitors demon hybrids, and was friends with Nanaya Kiri. However, he has since retired, and is currently some sort of shady back-alley doctor.
Since he knows the true nature of Shiki's anemia, he has been maintaining his health with Eastern medicine. It's not a cure, but that's the beauty of it. Of course, Shiki's anemia was never something that could be cured in the first place.
According to Shiki, he's a mad doctor. Though he is foul mouthed, he is also possessed of a strong sense of duty, and it seems he is acquainted with Miyako. Probably, she used to tag along with Shiki when he visited the Jinan Clinic while he was living in the Arima household. As his appearance suggests, he has an interest in Chinese martial arts, and may even be the one responsible for teaching Miyako the basic forms.