Shinra Kishitani stays mostly indoors and wears a white coat even when not working as a doctor. He went to the same primary school as Shizuo Heiwajima and has since then been interested in his unlimited strength in times of anger. In fact, he rather enjoys when Shizuo makes use of his strength. His father was a doctor and, since childhood, he has participated in surgeries with his father, the most notable a vivisection on Celty.
He went to the same middle school as Izaya Orihara and has been friends with him since then. Later, in high school, Shinra introduced Izaya to Shizuo and from the first moment they met, a murderous relationship began between them. He enjoyed to some point the constant fights between Shizuo and Izaya but at the same time he blames his two friends for ruining his high school days, saying that because of them he got much better dealing with broken bones and stitching wounds.
Shizuo was violent and Izaya was shady as hell, so no girls were willing to approach them. About Izaya, he thinks that he's annoying, but he can't really say that he dislikes him. In his opinion, Izaya is nothing but true to his own desires, in the same way Shizuo is true to his emotions. Though in Izaya's case it would have been much better had his desire been just money or women, but on the contrary, he has no interest in either of these things. His only passion is what he calls "human observation".
As a doctor, Shinra is rather interested in what he finds as interesting phenomena, such as Shizuo's power and Celty's body functions. It is later revealed that he performed plastic surgery on the body of Mika Harima to make her head look like the head of Celty. His reason was to keep Celty from finding the head so she could stay with him. Shinra seems to have the capacity to tell what Celty is feeling, much to her surprise since she doesn't have a head to show her emotions. It has also been noted that he is in love with Celty, and intends to marry her. [Source: wikia]
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