Damon is the main antagonist in Black - Victini & Reshiram / White - Victini & Zekrom.
He is a descendant of the People of the Vale and wishes to restore his people's kingdom to its former glory. To this end, he has gathered allies among other modern-day People of the Vale and is in pursuit of Victini, hoping to use it to reactivate the Sword of the Vale, the ancient castle at the center of Eindoak Town.
Depending upon the version of the film being viewed, aspects about Damon change. In Victini and Zekrom, he owns a Reuniclus, while in Victini and Reshiram, he has a Gothitelle. This mirrors the games, in which Reuniclus, along with its evolutionary relatives, are only available in Pokémon White Version, while Gothitelle, along with its evolutionary relatives, are only available in Pokémon Black Version.
Three years prior to the events of the movies, Damon traveled from Eindoak Town to communities of other descendants of the People of the Vale, hoping to garner support for his goal to revive the Kingdom of the Vale; he was soundly rejected at every turn. This dream had been instilled in Damon at an early age when he traveled to the ruined site of the Kingdom of the Vale with his mother Juanita, where she voiced her own desire for the idea of repopulating the area.
Sometime after that, Damon managed to convince Mannes to support his plans. The two of them were hiking in the caves beneath the Sword of the Vale when, inexplicably, Damon found himself on a path Mannes could not follow. Continuing on alone, Damon discovered the Dragon Stone containing either Reshiram or Zekrom, and after revealing his goals to the dragon, became its ally. With the power of Reshiram or Zekrom on his side, Damon went back to the communities he'd visited before, and now was able to gain the support of the people in those towns.
After returning to Eindoak Town, Damon met up with Mannes again, as well as Juanita, his younger sister Carlita, and Ash as well as his friends. In a private meeting with his family and the town's mayor, Damon revealed his plans—borrow the power of Victini and use it to pilot the Sword of the Vale back to the Kingdom of the Vale, where the rebuilding of the town will become possible.
Later, using his Sigilyph, Damon begins to awaken the castle, activating the Protecting Pillars surrounding it. With Sigilyph's power, he contracts the barrier of the Protecting Pillars to force Victini into the castle, then traps it in a device that funnels its power into the Sword of the Vale itself. The castle begins to ascend, all according to Damon's plans, but he is soon confronted by Ash, Iris, Cilan, Juanita and Carlita, who all see the torture he is inflicting on Victini. Even Juanita is horrified - stunning Damon - and sends out her Golurk to make a desperate bid to stop Reshiram or Zekrom, but fails. Everyone else, meanwhile, is held in check by Damon's Reuniclus or Gothitelle.
After seeing a vision of the King of the People of the Vale warning Victini to never let the castle be moved again, lest the destructive "Dragon Force" it plugs be unleashed again - exactly what is happening outside of the view of Damon and his dragon - Ash manages to discover and ally himself with the opposing dragon. The two fight each other viciously until Ash's dragon begins to gain the upper hand, at which point they both fly beneath the clouds and discover the chaos Damon's actions have begun causing.
At this point Ash has managed to stop Reuniclus or Gothitelle, but cannot break through the machine to reach Victini. Damon's dragon soon arrives and destroys the machine, freeing Victini, then both Reshiram and Zekrom blow away the clouds so Damon can see the consequences of his actions. Remorseful and desperate, Damon asks the dragons to help him stop the damage, so they begin directing the Dragon Force back up into the Sword of the Vale itself. While Carlita and Iris evacuate on her Hydreigon and Cilan and Juanita board Mannes's helicopter, Ash refuses to leave unless Victini can be freed, and Damon intends to stay until the damage is repaired.
The castle, however, begins to go into overdrive and launch higher into the atmosphere. At the chaotic speed the castle is flying, Damon is thrown off the side of a balcony, presumably to his death; however, he is actually saved by Juanita's Golurk and returns to try to rescue Ash. He is powerless until Victini uses V-create to destroy the Protecting Pillars once and for all, at which point he reenters the castle and manages to give Ash a blanket.
After the castle is guided back down and landed at the point where the chaotic Dragon Force was flowing from, Damon collapses, overwhelmed with guilt for his actions. He is reprimanded by Juanita, who tells him that they have a lot of work to do if they are to rebuild the Kingdom of the Vale, and cheers up, parting with Ash with a handshake. He is last seen with his family and others beginning the work of rebuilding in the new location of the castle.