Ensign Syl is an alien officer of the USS Enterprise who conceals part of the ancient weapon called the Abronath inside the folds of her head in the film Star Trek Beyond.
Syl appears in Star Trek Beyond, set in 2263, and is played by Melissa Roxburgh. The back of her head is made up of ridged, finger-like growths that can open and close.
When Krall's swarm tears the Enterprise apart over Altamid, Kirk gives her the Abronath fragment to hide, and she tucks it away inside her skull. She is taken prisoner with Sulu, Uhura and much of the crew.
Krall learns where it is and threatens Sulu, and Syl gives the piece up to save him. Krall then puts the completed weapon to the test on her, and she dies as its swarm consumes her.