https://www.cmoa.jp/title/287310/

Louis is a prodigy who serves as a court magician and possesses talent that surpasses even adults. Living alone with his frail younger sister, he one day captures a fairy in the forest. That fairy is actually Eleanor, a girl who reincarnated into another world as a fairy on the hero’s side in this otome game. Small enough to fit in the palm of a hand, she is forced to live inside a cage. However, by drinking Louis’s blood, she can transform into a human-sized woman… Louis pours his obsessive, unhealthy love onto Eleanor. Despite his beautiful appearance and promising future, what is this boy’s true purpose in this world?