User blog comment:B214/Volume 22 spoilers/@comment-78.8.248.219-20161004044305/@comment-29963364-20161005203320

Before Regulus revealed his identity as the Namean Lion within the Longinus, Regulus Namea, he had the appearance of a young blonde boy wearing a mask. In the final match between Rias & Issei vs Sariaorg and Regulus, the lion transformed to reveal it's true identity.

Also, Sairaorg never lets Regulus fight alone, since he said that if it's left alone it might go berserk. So he likely never actually let Regulus fight until the Rias vs Sairaorg Rating Game.