If you're looking for manga similar to Ai no Donor, you might like these titles.
A boy asks his robot companion Omni for help, but the robot keeps getting the context wrong.
A freshman in high school wakes up early, excited for another "tryst" with his favorite classmate. Unfortunately for him, daydreaming in the shower may turn out to be the best part of his day. His sister destroys his uniform, and his parents stopped sending him money. He even discovers that his "boyfriend" has been seeing another girl on the side this whole time. When everything seems to be at rock bottom already, the last straw breaks...
A brother treated his sister like gold, giving her items to make her beautiful, watching her grow older and letting her know how lovely she had become. Yet beneath the surface of his facade lies a deeper truth - the truth behind their relationship and what has happened...
A young girl asks a mailman who's able to traverse space-time to deliver a letter to herself 10 years in the future, but he didn't anticipate what he'd find there...