Empty Heart: False Love(2003)

empty heart: Itsuwari no Koi Dakara

Jun's unspoken love for Ryuuta is a secret to everyone... except Ryuuta's little brother, Takumi, who would love to have Jun to himself. Now in high school, Takumi is student, Jun is teacher, and Ryuuta is moving on with his life. Takumi offers himself to Jun as a substitute for his brother, but will it be enough to fill their empty hearts?

(Source: DramaQueen LLC)