In Persona 5/Royal, Seiten Taisei was Ryuji Sakmoto's secondPersona, which is unlocked upon reaching Rank 10 of his Confidant. Stat and Affinity-wise, Seiten Taisei in Persona 3 Reload isn't too different from his Persona 5 version. He has good Attack and Endurance, but he also has Zio/Electric Skills that can be used for Downing enemies that have an Affinity Weakness. Unfortunately, Seiten Taisei is also DLC-exclusive, meaning you'll need to purchase the Royal DLC Set for Persona 3 Reload before he can be summoned from the compendium in the Velvet Room.