Ryuu no Hanayome Okotowari

In a town with legends of dragons in its past, Sakura and Ren helped to keep the peace as "dragon people," descendants with dragon blood, with the power to control water. One day, Sakura released Kouka, a legendary dragon sealed in the mountains, to save Ren, who was born with a frail heart condition. Kouka's spirit took Ren's body as a host, and he forced Sakura to help fully unleash Kouka's power...? The start of a Japanese-style fantasy about the taboo relationship between a girl and a dragon!

(Source: G Fantasy, translated)