Guillermo Barrera was a mercenary who mostly operated abroad.
Guillermo was hired by China White to kill Malcolm Merlyn, under the order of Moira Queen. The moment he arrived in Starling City's airport with his helicopter, he was confronted by The Hood, and after a brief fight, was killed. After hacking his phone, Felicity Smoak identified his target. His death however, actually allowed or at the very least prevented the Undertaking from being delayed.
- Skilled hand-to-hand combatant: Guillermo showed to be quite a good fighter. His fighting skills were almost equivalent to that of The Hood's. During the fight he was able to easily cut down arrows shot at him.
- Master knife-fighter: He used many kinds of blades and deployed them in combat. He was able to use them in close combat.
- Master knife-thrower: He used many kinds of blades. He was able to throw them with great precision.
- Knives: Barrera carried a pair of knives which he used to fight against his opponents up close.
- Throwing Knives: Barrera carried a number of throwing knives which he used as a means of attacking from range.