"The pleasure's mine. Rogier's the name. A sorcerer, as you might have guessed."

You can meet him in Stormveil Castle for the first time, but if you don't, he moves to Roundtable Hold after you kill Godrick. After he moves, you can talk with him and tell him you killed Godrick, he gives you his +8 rapier as a reward, as he no longer has need for it. After you've talked with him at The Roundtable, you have to find his bloodstain in the hidden area behind a secret boss in Stormveil. Return to Rogier to talk about his bloodstain, and he tells you about the Black Knife Assassins. After that happens, go talk with Fia and she gives you a map to find Black Knife Catacombs, kill the secret boss in there to claim his item. After you defeat the boss, go talk with Rogier and give him the item, then reload the are. He will tell you to go talk with Ranni at The Three Sisters, behind Caria Manor, Ranni will not divulge details about her part in the assignation, so you have to return to Rogier. He then tells you to join her and become one of her vassals, after which he warns you that he will fall in deep slumber. After awhile you can find his set and his letter on his person, he will disappear after you reload the area.