The Regent will appear randomly, and will often leave after one turn. Keep attacking it with these threats until you get it knocked down and into a hold up. You will not be able to join a persona that is above your age, so be aware, however, that you will not be allowed to join you at a level that is not yours. The key with Treasure Demons is finding out their vulnerability and knocking them down to spark a hold up. The Regent is the first Treasure Demon you'll encounter, and the youngest. Despite being named after the diamond, Regent doesn't accurately portray its true world counterpart, as the true Regent Diamond is white and is a cushion cut, while the Treasure Demon is a round cut. The Fusing Treasure Demons system is similar to Element fusion in the Shin Megami Tensei series, allowing a persona to rank up or down within its own Arcana, making it possible for the protagonist to locate any missing personas in their compendium. Regent produces the Regent accessory, which increases Strength by three points and gives the wearer the Critical Rate up effect. Emperor Arcana, Treasure Demon Regent is the first persona of the Emperor Arcana and the first Treasure Demon the protagonist encounters. The Regent Diamond, also known as Le Regent and the Pitt Diamond, is a 141 carat diamond rated as the world's most brilliant and clearest diamond.

Madarame persona 5 wiki