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|Appeared in|| X2: X-Men United|
X-Men: The Last Stand
Siryn is a mutant who can produce powerful screams from her vocal cords.
Theresa Cassidy was a student at Xavier's School for Gifted Youngsters. During William Stryker's attack on the mansion, her scream alerted the rest of the school that they were under attack. She was captured, but was rescued by Colossus.
Theresa was one of the students at Xavier's funeral.
- Black Tom Cassidy - Uncle
- Canon (2 films)
- Her name also appears on a list of names Mystique scrolls through on Stryker's computer while looking for Magneto's file.
- In the comics, Sean Cassidy is her father. However, in the films, Sean died before 1973 making it near impossible for him to be Theresa's father.
To be added