Vince and Nico were Lady Rhea's schoolmates during their early days in Thorolund. Now they, along with Petrus, are her guardians in her Undead pilgrimage to find the Rite of Kindling.
They wear the Holy Set of armor, "worn by the warrior clerics of the Way of White, who are famous for being unyielding in battle".
Later in the game, Rhea of Thorolund is found in a pit in the Catacombs that Patches pushes you into. Talk to her, and she’ll tell you that Vince and Nico have gone Hollow. Walk a short distance away from her, and you’ll confront them both. After you kill them, speak to Rhea once more to gain the Replenishment Miracle.