Alphamon

Alphamon is the Mega form of Dorumon, a Digimon of great power of the Alpha-Gain-Force. He is not just one of the 13 Royal Knights but also their leader of legend, destined to rule them yet condemned to be the "Lord of the Empty Seat," forever absent. Alphamon is also the only member of the knights who has his X-Antibody form the start, not being given or taken from a Digimon.

Digimon X-Evolution
When attacked by Omnimon, DoruGreymon Digivolved to Alphamon, regaining his memories. Omnimon immediately sided with Alphamon and took him to see Yggdrasil. But they faced DexDorugoramon, whom Alphamon managed to drive away before he entered Yggdrasil’s inner sanctum to destroy it but Dexmon appeared and a violent battle began. Powering up to Ouryuuken Mode, Alphamon attacked Dexmon - but he felt all attacks he inflicted on Dexmon.

This caused Alphamon to realize that Dexmon was his "other half", created from the data Yggdrasil removed from Dorugamon's body. Knowing this, he hurled his blade in a boomerang arc and grabed Dexmon as the blade returned, impaling them both. Alphamon transferred his X-Antibody to Omnimon before merging with Dexmon and regressing back to Dorumon.