Theresa Cassidy
Alias(es) Siryn
Appeared in X2: X-Men United
X-Men: The Last Stand
Status Alive
Actor Shauna Kain

Theresa Cassidy, also known as Siryn, is a mutant who can produce powerful screams from her vocal cords.


X2: X-Men United

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.

X-Men: The Last Stand

Theresa was one of the students at Xavier's funeral.

Powers and Abilities


  • Super Sonic Scream: Siryn has superhumanly powerful lungs, throat, and vocal cords, she can produce a sonic scream for various effects; Siryn's sonic screening was powerful enough to incapacitate William Stryker's strike force long enough to alerted the rest of the school that they were under attack.



  • 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 supposedly died before 1973 making it near impossible for him to be Theresa's father.


To be added

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