A Leader-in-Training is a type of Trainer that debuted in Generation IV. So far, they have only appeared in Pokémon Battle Revolution.
They are dressed in Pokétopia-style outfits which can be unlocked and put on Custom Pass characters. However, these characters will bear the title Pokétopia Supporter instead.
They are always training to be the next Leader of a Colosseum. Some of them tend to ask their opponent a question, only giving them the answer if the opponent wins.