Also known by QB, Kyubey is one of the main characters and later revealed as the main antagonist from Mahou Shoujo Madoka Magica.
They are a "Messenger of Magic" (魔法の使者 mahou no shisha) who grants the wishes of young girls, but in exchange contracts their services as Puella Magi (魔法少女, mahou shoujo, lit. Magical Girls). Girls who form a contract with them are tasked to defeat Witches -- bizarre entities that feed upon the hopes and dreams of normal people.
However, unbeknownst to the Magical Girls, Witches were actually a corrupted form of Magical Girls who were either killed in battle or eventually have their Soul Gem tainted over time after consecutive encounters with Witches. Kyubey intentionaly withheld this specific detail to create a millenia of cycle of war between Magical Girls and Witches in order to harvest the magical energy of these clashes for the benefit of their race. Their indifference to death and suffering upon the revelation of this fact upon Kaname Madoka has led to fans creating numerous hate arts subjecting them to various physical abuses.