So, I have a this is semi crack theory. Ulf the White is directly related to Viserys. He is either Viserys’s younger bother or an eldest son. So, why do I think that it is because both Ulf and Viserys share a strange similarity in looks and personalities traits. Considering how good casting is in this series, I am doubtful that this was accidental. They share similar personalities; both Ulf and Viserys are people that find joy in food and drink, party and friends. Ulf knows nothing of his histories; he makes them up as he goes while Viserys is obsessed with their family history.
To me they are two sides of a coin, representative of their respective classes. I would like to image if they to ever meet that they’ll got on quite well, for a little bit at least.
I have two versions. Then he was telling his mates at a tavern that he is son of Baelor the Brave but later on confessing to Hugh and Adam that he has no idea who his father is. What if it was accidentally true statement?
The second version is what if Ulf is Viserys bastard son. In season one Daemon and Viserys had mentioned how both would venture into city, to drink and find entertainment in brothels, then they were teenagers/young adults. Presumably before V's marriage to Aemma. If it there so, that’ll make an interesting plot point. Viserys in perused for a son had destroyed his wife. Him neglecting all of his children, including Rhaenyra, (despite being his “only child” he had neglected to involve/prepare her for the throne.) doomes them all to die in same war. As Dance of dragons is a tragedy it would make sense.
Despite me writing out theories, then I think of those similarities I like to consider as not a big plot twist but a fun easter egg of sort from production team, not something that must be stated outright.