Kyle is an American with a military background, and he obsessed over Jacosta, which Adrian exploits.
As with most of the Red Pill cult, he believes in Adrian for most of the show. After learning that Jacosta is a boy, he attacks Jacosta and kills Mr. Adams, but shows restraint towards Leila, and instead abducts her to chase down Tess.
Leila has no choice but to go along with the honest murderer, but upon seeing his mental instability, she decides to leave him behind. This seems to solidify his devotion towards Adrian, and he comes back later in full swing -- attempted murder of Tess. Leila intervenes, and ends up stabbing Kyle multiple times, though his death is unconfirmed
