Doudou

His mother was injured during the battle between Gods and Devils which resulted in Doudou being inherently weak. When he was born he was only a 1/3 of the size of a normal young dragon because the devil energy from his Mother's injury was passed to him.

Once the Dragon God was resurectted he personally did the blood transfusion on Doudou to remove the devil energy from his body. Once it was completed his appearance changed to that of a teenager.