Koishi Gatekeepers Series, Book 1.

Never make a deal with a dragon. Sandra's sister is dying. Doctors say nothing can be done, but she refuses to give up hope. She has depleted all their resources searching for a cure, when a little bird whispers in Sandra's ear about secret worlds, gates, and the keepers who protect them. A dragon gatekeeper, who hoards magical treasure within his volcano home in Japan, possibly owns an item capable of curing her sister. Gates choose their keeper...