Theresa Rourke Cassidy

Theresa Rourke Cassidy, the Irish-born daughter of Sean Cassidy, inherited her father's powerful sonic abilities. Operating under the name Siryn, she carved out her own heroic identity and later became a central figure in X-Factor Investigations.

Theresa Rourke Cassidy stepped onto the Marvel stage in 2007, born from the sharp, character-driven pen of Peter David and the dynamic art of Khoi Pham in X-Factor #20 — a Modern Age debut that planted her firmly in one of that era's most acclaimed corners of the X-universe. Her appearances are concentrated in the compelling X-Factor corner of Marvel, where she keeps genuinely memorable company: Strong Guy, Monet St. Croix, Multiple Man, and the rest of the colorful, complicated cast that made this series a fan favorite. With 22 catalog appearances spanning titles like X-Factor: Second Coming and X-Factor: Invisible Woman Has Vanished, she's a character whose presence carries real weight in a tight-knit, richly written world — exactly the kind of figure that rewards readers who dig deeper into Marvel's Modern Age.