Wesselen is a prelate within the Order of the Radiant Heart in Athkatla. In effect, he is the order's leader.
In Baldur's Gate II Chapter 6, you can speak with the prelate and ask him to help you in the fight against Bodhi. He will agree to help you and will send a group of knights to fight alongside you in the Lower Tombs of the Graveyard District.
He can also be spoken to during Chapters 2, 3, or 6 as part of removing the Solamnic Knights from the Planar Sphere.