Sean Cassidy

Few Marvel characters can claim a Silver Age debut and nearly six decades of continuous relevance, but Sean Cassidy — introduced in The X-Men #28 back in 1967 by Roy Thomas and Werner Roth — has done exactly that, racking up 359 catalog appearances across a run that stretches all the way to 2025. A proud X-Men member, he's spent his comics life in genuinely elite company, sharing adventures with Wolverine, Cyclops, and Ororo Munroe across the landmark pages of Uncanny X-Men and Classic X-Men. With 43 key-issue appearances to his name, collectors have long recognized that Sean Cassidy isn't background color — he's a fixture of Marvel's mutant mythology, the kind of enduring character who reminds you just how deep and richly layered the X-Men's world truly is.