Everything that will be said below is information invented by the author in accordance with the details from the official wikipedia of the game. Do not take what is said below for the truth

Based on his wiki history , I believe he would look like a fairly skinny kid with medium length hair, slightly narrow eyes, and a fencing enthusiast. This is because he was born in the Painted World of Ariandel before the painting was infected. In that picture, a winter storm is raging, which is why his eyes had to be protected from the wind and snow, and he is thin, since it was always difficult to find food in such a storm. My hair was so long because I had to dress as warmly as possible to stay warm. And he looks cute because he actually lives in the quietest part of the world. The Painted World of Ariandel is an oasis for tormented souls. People entered Ariandel to finally calm down these torments. And for these people, Sulyvahn was a stranger, he was born into this world, he lost nothing, did not feel pain. And although they despised him in the picture, he endured, because they had seen a lot in their lives, but deep down there was resentment. He loved to hone his fencing skills. He learned something from the good people who came to Ariandel, but he was mostly self-taught. There was no one to fight here anyway.

But at some point, when despicable people again began to mock and envy Sulyvahn, he decided to leave Ariandel. So he began to wander around the kingdom of Lothric. Thus, he found himself in the desecrated capital, where the desecrated fire raged. This flame released long-pent-up anger and resentment towards the new arrivals to Ariandel. He enchanted his blade with flame and called it the sword of mud. The flame itself consumed his good side and released the bad. In his travels, he reached the Temple of the Depths, where he met adherents of the Cult of Aldrich. By that time he had already swelled and became liquid, but with intelligence. There, Sulyvahn and Aldrich came to the conclusion that they had fairly similar goals. Sullivan wanted revenge, and Aldrich wanted to eat more human flesh. Over time, as Sulyvahn grew up, he learned the magic and dark wonders of the Temple of the Depths. There, upon completion of his training, he was given the sword of justice, enchanted by darkness. The time had come to carry out the revenge that Sulyvahn lived on. He burst into Anor Londo and went straight to the ruler. At that time, it was Gwyndolin, who had weakened over time (we remember that we beat him in the first part). He locks him in the same battle room with Ornstein and Smough and calls Aldrich. The cult of the Temple of the Depths and Aldric himself has penetrated Anor Londo, Sullivan locks Aldrich with Gwyndolin. He locks one of Gwyn's supposed daughters, Yorshka, in a tower and claims the maid as his dancer. Aldrich becomes the devourer of gods. Sullivan captured Irithyll, proclaimed himself Pontiff and began to rule Irithyll. But this was not enough for him. The anger that took over Sulyvahn's mind forced him to begin experiments on the inhabitants of Irithyll. As the flames began to die, Sulyvahn called upon Knight Vordt and a new recruit, who was formerly a dancer, to travel to Lothric to stop the Unburned from taking his and Aldrich's power. He gave them rings that disfigured their bodies and minds, allowing him to control them without orders.
Pontiff Sulyvahn, the usurper, is a disgusting bastard who should not be pitied, despite his fate.