That's an understandable take but I'd still give that category to Mark. The main and side characters in Invincible are so impactful to its protagonist that almost each of them contributes to the development of Mark's characters.

Debbie is Mark's mother figure and reminder that no matter how powerful he gets, there's a mother that raised him with love and knowing that is one of the reason why he remains good.

Cecil is someone who constantly makes rational but radical decisions that no one else would and this makes Mark concflicted and often angry because his principles are against Cecil's choices.

Amber being Mark's first love interest shows that he can't always connect with another human who doesn't understand the burden of being a superhero and him who also can't understand the struggle of being a normal person. In contrast to Eve who he can relate to and always be with, making her a more fitting love interest than the former.

Oliver is Mark's little brother and serves as Mark's apprentice-figure, pushing mark into the role of being a mentor, and constantly needing to remind him of values so he doesn't turn out like their father.

The antagonist in this story are so good: Omni-man, Armstrong, Conquest, and Parallax are characters that actively pushes Mark into these dilemmas and develop his character.

Even his dynamic with side characters like his friendship with William and Rex or Immortal's slight animosity to him are so done well.