Nascour first met Wes and Rui at Phenac City. Nascour was leaving the mayor's house. No explanation was given at the time as to why he was there. He promised Wes that they would meet again soon and he spoke truly.
At Realgam Tower, Nascour forces Wes to battle against four trainers in a row. He then fought Wes himself, in front of a packed stadium of witnesses. However, Nascour failed to defeat Wes. He then revealed that he had been taking orders from Es Cade, the mayor of Phenac City, who now called himself Evice, all along.
Nascour is presumably arrested along with Evice in the finale of Pokémon Colosseum.