Vikings is a series where alliances are made and broken shockingly quickly, where romances are often cut short or complicated, and where even families are broken up and take sides against each other. In the desire to raid and conquer, it's difficult to maintain real, loyal friendships - even those that seem the most secure, like Floki and Ragnar's friendship, struggle against the clash of religious faiths and desires to rule.
Despite this, there are some friendships in the series that are loyal and constant, and that make perfect sense. Some are introduced as friendships, and others grow in a natural and predictable way throughout the series. These, though, were the friendships that no one would have expected when the characters were first introduced to each other.
Athelstan & Ragnar
One of the most loving and devoted friendships in the series was also one of the most surprising, at the start. Ragnar kidnapped Athelstan (a monk) when on his first raid to England, and brought him back to Kattegat to keep and enslave. At first, Athelstan desperately attempted to keep to his faith and God, and saw Ragnar purely as a captor. Over time, though, the two became incredibly close, and Athelstan became a Viking, a free man, and Ragnar's confidante. When they first met, though, the assumption was that Athelstan would always hate Ragnar for kidnapping and enslaving him, and that Ragnar, a true Viking who wanted to raid England, would always see Athelstan as less than equal.
Torvi & Lagertha
When Torvi and Lagertha first met, it was as enemies. Torvi was the wife of Jarl Borg, who battled Ragnar, and was eventually blood-eagled by him. Of course, this was not at Lagertha's behest, but still, the two women started on opposing sides - but eventually, Tovi became Lagertha's right hand in Hedeby, joined her in retaking Kattegat, and stuck with her as a devoted friend and ally for the rest of her life. Clearly, their husband's feuds and actions didn't stand in the way of their own connection.
Siggy & Aslaug
Siggy was the wife of the first (in the series) Earl of Kattegat, Earl Haraldson. When he is deposed by Ragnar, she is left without a clear place in their society, and asks Lagertha for a place in her household. Then, when Lagertha is replaced by Aslaug, Siggy retains that place - but there is no love lost between the two. Over time, though, this grows, as Siggy becomes a vital part of the household, and starts to see how Aslaug struggles, and her weaknesses as well as her power. In the end, Siggy even died saving the lives of Aslaug's children.
Ivar & Dir
When Ivar fled to the Rus, he quickly became allies with Oleg, who wanted to join forces with Ivar (or rather, supply a force to Ivar, given that Ivar had nothing on arrival) and invade Norway. Oleg had his own issues as a ruler, though, and betrays his own brother, Dir, locking him up and torturing him.
Given that Ivar is a tyrant and a killer himself, and has similar desires to rule, his friendship with Oleg seemed like a perfect fit - but over his time with the Rus, Ivar changed, and fans were surprised to see that he eventually allied with Dir against Oleg, and passing messages with his men (as shown above).
Yidu & Ragnar
Yidu was another enslaved person that Ragnar formed a more meaningful relationship with - although it's debatable whether or not they could truly be considered 'friends'. Ragnar treated Yidu well, giving her a 'home' and space, talking to her about her own home, and even telling her she could be free - and the two had a real connection, of sorts. Once again, this was surprising - not least because she was a captive from Frankia, and hated Ragnar and the Vikings at first, but also because he shunned many of his own friends and peers in order to spend time with her. Given that she did introduce him to addictive drugs, and that he eventually killed her, this friendship was not a simple one.
Lagertha & Heahmund
Lagertha and Heahmund were lovers, rather than platonic friends, but it was still a connection that shocked fans, as Heahmund was introduced as a Bishop who was obsessed with killing 'heathens', and Lagertha is, obviously, a 'heathen'. Their relationship also seemed to come out of nowhere, as they went from battling each other to being sexually involved incredibly quickly - and while this isn't out of character for either of them, the suddenness of the connection was a shock to many.
Floki & Ivar
This may be one of the least odd friendships here, as both Floki and Ivar were outsiders in Kattegat in many ways, and Floki was one of Ragnar's closest friends. However, it was also strange to see the depth of their closeness, as Floki often lived outside the main area of Kattegat, and early on, was intentionally not particularly easy to get to.
For a boy who had to drag himself across the ground to get anywhere, and who has an anxious mother who constantly worried for him, it's surprising that this became such a regular connection for the two of them.
Ubbe & Lagertha
Lagertha was ousted from her marriage and her throne by Aslaug, and then murdered Aslaug in revenge, so it's no surprise that she doesn't exactly get along well with most of Aslaug's sons. Ivar, in particular, spent his life seeking his own revenge for his mother's death. Ubbe, however, surprised everyone by joining forces with Lagertha, and eventually becoming one of her closest allies. He (and Lagertha) were able to see that fighting was not going to cause anything but more pain and trouble, and that putting past grievances aside would be best for everyone.
Ragnar & Ecbert
One of the most complicated and enduring friendships in the early seasons of Vikings was between Ragnar and King Ecbert, who brokered and broke treaties, and yet who seemed to truly care for each other at times, too. It was always going to be a surprise to see two Kings, one invading the other's country, become not just allies, but friends - especially when their religious beliefs were also an issue of contention. However, between Ecbert and Ragnar's mutual curiosity and love of learning, they actually became friends, albeit friends who battled and imprisoned each other from time to time.
Ivar & Igor
For almost the entire series, Ivar is set up as a true villain. He is vicious, cruel, an incredibly talented fighter, and a casual murderer. He is brilliant, but utterly ruthless, and kills his own brother for a public insult. He becomes a tyrant, convinced he is a god, kills his own son, and then murders his wife before fleeing Kattegat - so it was a true shock to see him develop an almost fatherly relationship with the young Igor while with the Rus. The two seemed to become true friends and allies, too, and unlike most of his other relationships, Ivar didn't seem to want anything but the best for Igor - even if it put himself at risk. This was phenomenal character development for Ivar, but surprising, too.