Ecbert begins his quest for power as a member in the court of Emperor Charlemagne. He proves himself to be an expert tactician, winning battles and even capturing Ragnar's brother, Rollo (Clive Standen) at one point. While he's a fierce opponent of Ragnar, the two have something of a mutual respect for one another. This is seen on full display when the two have conversations together, and it's these scenes fans have really latched onto over the years.
Redditor /u/DanyeeIsAnulmint writes, "Their chemistry as actors was phenomenal and they really complimented one another in all of their scenes." An example of this kind of scene occurs in a season 4 episode when Ragnar is captured by Ecbert's forces. Ecbert demands his men release Ragnar, even though he believes that means Ragnar will kill him. The admiration the two have toward one another is evident, and it makes it clear Ecbert isn't just a bloodthirsty villain. There's genuine humanity behind his political aspirations.
Other comments seen throughout the Reddit thread support this notion, with people saying things like, "I loved their banter," and, "Both [Ragnar and Ecbert] are great and ambitious thinkers, they also look further [than] most of the people around them which makes the two really special." Redditor /u/EscapeFromTLH makes the fair distinction of, "I think he's the second best character, for sure. I can understand why he's not the second-most-liked character, though." While Ecbert has a friendly forward-facing personality, it's only a facade to hide his true intentions. He's a backstabber when it comes right down to it, but that kind of complexity is precisely the reason why people tend to love villains just as much as heroes.
Ecbert may no longer be on Vikings, but fans will still want to tune in when the second part of season 6 airs to see the conclusion of this epic Norse tale. While an official release date hasn't been announced date, it should come out at some point in 2021.