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Names of shit are whatever.
Slightly overpowered final gym leader (sup pseudo-legendary) based on the idea of Gym Leaders being more relevant to plot and an extant post-game.
The first gym in the region is Dark type, and the champion is a rival character that began to train Dark types after being inspired by a huge loss to said first gym. The final gym leader, a Psychic champion, is the previous champion who bemoans losing to a Dark-type trainer.
Dialogue while fighting the Elite Four suggests the previous champion was actually forced away from his position because of more than just losing a battle against your rival, and post-NPC dialogue explains that cheating as using legendaries.
You eventually meet the gym leader again and in battle he finally reveals that he was a disgraced champion for relying on overpowered Pokemon and decides to show you the power of his Jirachi in battle.
When you defeat him he helps you find one of the Lati@s (game dialogue decides which one it is, sort of like in Emerald, or else the version dictates it) and then uses the other Lati@s against you in a battle later on.