Gabriel Agreste (aka Hawk Moth) is Adrien's father and a well known fashion designer in Paris. However unknown to everyone even his own son, Gabriel is also the villain Hawk Moth who can possessed people down on their luck using butterflies called akumas. The reason being that he can get the Ladybug earrings and Cat Noir ring which combined, can grain untold power. His motive behind this is because his wife and Adrien's mother Emille Agreste went missing while on a trip in Tibet and using the Miraculouses would bring her back.
In "The Collector", in order to avoid the suspicion of being Hawk Moth, he akumatizes himself into the Collector, a supervillain who has the power to imprison any person or object within his magical notebook.