Commander Yammark is a dragonfly-themed Reploid and one of the eight Nightmare Investigators in Mega Man X6. Originally created by Gate for the purpose of overseeing a nature preservation project, his project was sabotaged by Gate's disgruntled colleagues and Yammark himself perished after his flight systems malfunctioned and he crashed. He was resurrected by Gate years later as a Nightmare Investigator, but was paranoid about Gate and the Maverick Hunters turning against him.
- Yanma Option - Yammark summons a swarm of robotic dragonflies to surround and protect him. These dragonflies are weak, however, and can be easily destroyed with any weapon. The dragonflies will also shoot energy blasts. Both X and Zero can copy the Yanma Option.
- Formation Guard - Commander Yammark will launch the dragonflies at the player, like missiles. They can be hit and deflected back at him. He will take slight damage and will be stunned if hit.
- Formation Wing - Commander Yammark will level out with the player, then fly in a straight line towards the other side of the screen, while the dragonflies fire straight ahead.
- Formation Final:
- When Commander Yammark has 1/2 of his health left, he will call the dragonflies to line up behind him and fly around the screen, attempting to fill it up.
- Or Commander Yammark will call the dragonflies to line up above and below him, then he will fly from side to side, trying to hit the player.
Commander Yammark is the weakest of the eight selectable bosses in X6 and can be easily defeated with just the X-Buster/Z-Saber. However, he is also vulnerable to the Ray Arrow weapon and Rekkoha technique obtained from Infinity Mijinion.