Ryuuou no Oshigoto!

Kuzuryuu Yaichi is a prodigy shogi player who won the title of Ryuuou at the age of 16. Following his victory, he has been in a slump until he is approached by Hinatsuru Ai, a 9-year-old girl who begs him to make her his disciple. Astonished by Ai's potential, Yaichi agrees to become her master, and the two then brave themselves together in the world of shogi with their friends and rivals. However, this comes with the condition imposed by Ai's mother that Yaichi is to marry Ai if she does not become a major player before graduating middle school. [Edited from Wikipedia]