When it comes to The Vampire Diaries, although the story seemed to begin as a tale about the unusual relationship between Damon Salvatore, his brother Stefan, and his brother's girlfriend/their ex's doppelganger Elena, the show and story evolved dramatically over time.
Given that eight year evolution throughout the series, it's hard to say which character truly deserves the title of main character of the series. Arguments could be made for Elena Gilbert, cases could be made for Stefan Salvatore, but ultimately here are all of the reasons why Damon Salvatore reigns as the central figure in the entire Vampire Diaries saga.
Because He Was The Only "Pure" Vampire In The Series
The show is literally titled The Vampire Diaries, so given the fact that Damon was the only straight up vampire throughout the entire series, it seems fair to say that he is the lead character.
Elena was a human doppelganger and became a vampire, Stefan was a vampire for the entire run of the show but was also a doppelganger, so Damon was the only lead character who was only ever just a vampire.
Because Ian Somerhalder Was The Face Of The Show
Ian Somerhalder was arguably the most famous member of the cast before the series actually started, so it makes sense that he was used in the promotion most heavily. But even once the show became a hit, he seemed to be the primary face of the series, and that continued for the entire run of the show.
Even those who didn't watch it may have known that Somehalder was the star, which seems to indicate that Damon was the central figure within the show as well.
Because Ian Had The Biggest Fanbase
Somerhalder may have had a head start in comparison to the rest of the TVD cast when it came to fame, however it's safe to say that the success of the show rocketed him to stardom on a level that it didn't for the rest of the cast.
Most of the actors understandably had a huge image boost from it, but Somerhalder clearly benefitted the most from it.
Because Damon Had The Biggest Fanbase
Putting aside the popularity of the actor who played Damon, it's also readily apparent that Damon Salvatore had the biggest fanbase out of all of the characters in The Vampire Diaries.
It's not a hard and fast rule that the main character of the show also becomes it's most popular, but the fact that Damon's overall fandom seemed to outgun both Stefan and Elena's individual fanbases combined says a lot.
Because His Love Story With Elena Lasted From Start To Finish
In fairness, Elena was head over heels for Stefan at the outset of the show. However, it was very clear from the moment that Damon and Elena met that they had a romantic future ahead of them, and it was that romance that took up the bulk of TVD's storytelling.
Damon and Elena's love story was the show's longest, it's most important, and it ultimately wound up being endgame.
Because His Love Story With Stefan Lasted From Start To Finish
Obviously the love stories within The Vampire Diaries were the major focal point of the show, but family was a huge overarching theme as well.
And Damon is clearly the main character of the series, because once his love story with Elena had to be put on the back burner on account of Nina Dobrev's exit from the show, his relationship with Stefan took a front seat, and it was a consistent story within TVD for the entire show as well.
Because His Character Development Was The Most Consistent Development In The Series
Stefan and Elena are both great characters in their own right, but honestly, their character arcs didn't go quite as far as Damon's did.
Stefan and Elena may have both gone through great changes within the story, but the Stefan and Elena that viewers saw at the start of the show aren't that drastically different from the Stefan and Elena that they saw at the very end. However, Damon was an almost entirely different person.
Because He Survived The Finale
Another reason why it feels like Damon was ultimately the main character of TVD is because even though the show came to it's end, his character's story did not.
He got to live a long and happy life with Elena and then finally reunite with his loved ones after he died, presumably of natural causes. Damon is the character who got everything he ever wanted in the end.
Because He Got The Girl
A huge aspect of Stefan and Damon's rivalry was at first developed because they were both in love with Katherine, and it then intensified when they both fell in love with Elena.
Elena's choice between the Salvatore brothers was a source of tension for years, but in the end, Damon was the one she loved the most. That again would indicate that he is the real main character of the series.
Because His Reclaimed Humanity Seems To Have Been The Point Of The Show
It's hard to narrow down the purpose of an entire TV series, especially one as long-lived as The Vampire Diaries.
However, given the way that the show started off and the way that it ended, it seems like Damon getting back the humanity he'd lost was a huge aspect, quite possibly the defining aspect, of the entire story that was told throughout the show.