I saw a lot of people ask this after finishing 'Red Queen'. Maven is the younger son of Queen Elara and King Tiberias, and Cal's brother. The thing is, you spend the whole first book thinking he's the sweet, clever underdog who gets Mare, while Cal's the golden boy. Then the ending of 'Red Queen' hits you like a truck. He was in on it the whole time. His mother's mind manipulation, the betrayal... it's not just a twist, it redefines the entire series. He's the main antagonist afterward, but he's so tragically shaped by his mother's interference that you almost pity him. Almost. His obsession with Mare becomes this terrifying, corrosive force that drives the plot of 'Glass Sword' and 'King's Cage'. The complexity is what makes him stand out more than a typical villain.
I've seen some readers argue his character gets a bit repetitive in his later appearances, stuck in a loop of obsession and self-destruction. I get that, but for me, watching a character who was fundamentally broken from childhood wield so much power and be so utterly hollow inside is more compelling than any battle scene. The chapters from his point of view in 'King's Cage' are brutal.