The Invention of Skiing

Question: So, Uller is Sif's son and Thor's stepson... so who's Uller's dad?

Nobody knows, actually. While Uller is clearly said to be Sif's son in Gylfaginnig, he's also clearly noted not to be Thor's son, with no mention or explanation of who his actual blood father might have been. As if to drive the point home, Thor's referred to several times by the kenning "stepfather of Uller", so it was clearly an established fact that he wasn't actually related to the kid. But none of the Norse myths we still have mention anything about Sif's life or possible spouses before Thor, nor about Uller's family history, so where did our favorite Odin-impersonator come from?

Technically, you could decide that any of the other Norse gods were Uller's father, depending on what kind of shocking reveal you want in your story. Is he another son of Odin (which would maybe explain why he did so well taking over for him that one time)? Is he a son of Loki, who (at least according to himself) is famous for seducing the ladies of Asgard, including Sif? Is he, like Thor, half-giant with some towering jotun in his mother's past, or is he the unsung son of someone like Tyr or Heimdall? And, of course, this is Scion we're talking about - a goddess with a famous son with totally unknown father sounds pretty familiar, and Uller might be that rare example of a god who was himself a Scion and ascended to godhood in times long gone. Another theory floating around the scholarly community is that Uller might be the son of Aurvandil, the enigmatic and little-known star-god that Thor cut the toe from (thus offending his surgeon into refusing to remove the rock in his head), pointing out that he is, like Uller, a skilled archer with ties to Thor's family.

We're particularly fond of the theory that the ladies of the pantheon are mostly Vanir brides, taken by the Aesir when the war between the two peoples ended. Just as Njord was married to his Vanir sister and had his children Freyr and Freya with her before coming to live with Odin's brood, so Sif might have had a Vanir husband before she was taken to Asgard, making Uller also one of the true Vanir among the Aesir.

Or you could even get totally crazy and decide that his father was someone not even among the Aesir, making him the son of an Inuit or Siberian god or even someone from a faraway pantheon whose identity remains secret because of Super Secret Plot Reasons. There's no guidance from mythology in this case, so if you've got a plot in mind, go wild!