The doctor that is in charge of Rika's care. He used to take care of her at another hospital but was transferred to the one in Yūichi's town. Rika then transferred to the same hospital so that he could still be her physician. He harbors some feelings for Rika and he tries to sabotage Rika and Yūichi's relationship claiming its for fun. In reality, Natsume is doing this out of his own experience with his wife who died from a similar condition to that of Rika and possibly doesn't want Yūichi to go through the same thing.
-Wikipedia