Fans of Viking times have been lucky in recent years, as several pieces of media have focused on the historical period. Two of the most famous examples are Assassin's Creed Valhalla and the Vikings TV series. While both took some creative liberties with real-world history, they each used known historical figures as characters.
They're far from the first historical figures depicted in the Assassin's Creed games or in popular streaming series, and several individuals are depicted in both stories. However, the two versions aren't exactly the same given the difficulties of depicting ancient historical figures.
Harald
The main similarity between Harald from AC Valhalla and the one from Vikings was their ambition, as both desired to rule and be Kings of their land. Vikings' version of the character had a multi-season arc surrounding his ambitions, whereas the Valhalla version was only in the prologue.
In fact, the only purpose to Valhalla's Harald was to take over the home of Eivor and Sigurd to give them a reason to leave and travel to England, where most of the game took place. He was a much bigger character in the TV show and seemed a crueler man as Vikings' Harald did a lot of questionable things throughout the show. The two characters didn't look very similar either, with their different color hair and the face tattoos on the Vikings' version.
Halfdan/Hvitserk Ragnarsson
Both depictions of this son of Ragnar were fearsome warriors, and that's about all they had in common. While they were technically supposed to be the same character, they couldn't have been more different in the two pieces of media. They didn't even share a name, with Valhalla's version being called Halfdan and the one from the show named Hvitserk.
Halfdan was a lot more self-assured and mentally tougher than the Vikings one. He was a true leader of his people, and he didn't suffer from all of the personal problems that plagued Hvitserk. Yet, he also didn't get the same amount of screen time. Hvitserk was a significant character in the last couple of seasons of the show, even if he was never quite as important as his brothers.
Rollo
Rollo was a skilled and powerful warrior in Valhalla and Vikings. He also boasted a large frame, which he seemed to like to show off because he was often shirtless—although the Vikings' version did wear clothing more often in the later seasons.
Yet, their personalities were very different. The Rollo from the game was much more joyous and promiscuous than his TV counterpart, and the character from the show was a more rugged and traitorous person. Their arcs were very distinct, too, as the Vikings' version did a lot more, from his early years where he had a constant rivalry with Ragnar, which was among the best in the Vikings series, to his time in Francia. The Valhalla version did spend time in Francia, too, but that was before the events of the game.
Alfred
Alfred, A.K.A. Alfred the Great, was usually depicted in each piece of media as a patient and fair ruler. The one from Valhalla was a bit more questionable morally, but neither version was truly evil. Unlike many of their people, both were willing to work with those from the north.
Their storylines were very different from one another, though. Valhalla's Alfred played a big part in Eivor's campaign against the Order of the Ancients. Alternatively, Alfred's arc from Vikings was all about his rise from a shy child to a true King of England. As a result, the Vikings' depiction was a lot younger than his video game counterpart, which explains their vastly different appearance.
Ubba/Ubbe Ragnarsson
The two versions of Ubba (Valhalla)/Ubbe (Vikings) were calm and collected individuals—at least compared to their siblings. Yet, their level-headed nature doesn't mean they were weak, as they were strong combatants who were no pushovers.
Their physical appearance wasn't too similar, however; the character from the show was noticeably leaner than his video game counterpart. Plus, their eyes and hair colors were distinct from one another. They had differing stories, too, as Ubba was often overthrowing kings in England alongside Ivar. In contrast, the Vikings' character featured in various storylines because Ubbe was arguably the main character of Vikings by the last season. Sometimes he was fighting wars, and, other times, he was traveling the world. He certainly didn't join forces with Ivar too often, as they were usually enemies.
Ivar The Boneless
Each version of Ivar was unhinged and had no qualms about killing anyone they wanted. The one from Valhalla was a little more playful, as he was willing to make jokes, even if the nature of them could be controversial. Ivar from Vikings was more ambitious, as he wanted to become the most famous Viking the world had ever known. In fact, much of Ivar's arc in the show was about gaining more and more power.
He did so despite his inability to walk, which didn't seem to be an infliction that the Assassin's Creed character shared. But, that wasn't their only difference visually, as the two didn't look very much alike due in part to the show version being much younger.