Undercover Assassin, Mata Hari

You can choose not to untap Mata Hari during your untap step.
Mata Hari has hexproof as long as it isn't attacking
{1}{u}: Tap target creature an opponent controls. Activate this ability only if Mata Hari is untapped.
{t}: Gain control of target creature an opponent controls for as long as Mata Hari remains tapped.