It's an ensemble show. There are so many important characters that you could easily argue than none are the main character. When everyone is special, noone is.

Ned made sense as a main character in season 1 given how much focus was on him. The main storyline was around the mystery of Jon Arryn's death with most other storylines being subplots. This turned out to be the start of the vast majority of storylines in the show from after Ned was killed and the kingdom was plunged into war.

Robb being this character's son out to avenge him made him inherit that slightly even though less of a focus was on him.

Arguably there waa no main character after Ned died, but there was definitely no main character after the red wedding.

So why split hairs about what classes as a main character? This hasn't been a show with a main character for years. In this case a "main character" can simply refer to an important long-term recurring character with arcs and storylines of their owns.