Wallace is unique (along with Iris) in the sense that he has played the part of both a Gym Leader, and a League Champion. Wallace is the Gym Leader for the Sootopolis City gym in Ruby/Sapphire, as well as Omega Ruby/Alpha Sapphire. However, in Pokemon Emerald, Wallace returns as the League Champion. As both a Leader and a Champion, Wallace continues to specialize in Water Pokemon. I think this is a bit unfortunate when he takes the position as Champion for the reasons explained below.
Traditionally in Pokemon Games, the Champion had a diverse group of Pokemon, rather than focus on one type over another. This is what makes a Champion so difficult to defeat, because much planning and strategizing is required to combat all types of Pokemon that the Champion may use. This puts Wallace at a bit of a disadvantage, as he specialized in certain types of Pokemon. But to be fair, Wallace uses mostly dual-types in Emerald, and not a single one of his Pokemon are limited to water type moves. For example, Wallace uses a Whiscash, which has an immunity to electric type moves. So you still have to do a bit of planning around his team (see the 'team' section for more information), but perhaps not as much as other Champions.
But if we are being completely honest, it would probably be pretty out-of-character for either Wallace or Iris to completely drop their specialized typing for the sake of being the Champion. Iris is unique in the sense that when she becomes Champion, she adds three different Pokemon to her party, and not one of those is Dragon type. There is compromise there, as she still keeps the Dragon typing with half of her party. It might have benefited Wallace a bit more to have done something similar. But as we know, Wallace is very dedicated to his Pokemon, and I think he has a really special attachment to the Water typing in general.
Wallace has received the Champion title not just in the games, but also in the Anime and Manga series. However, it was later implied in the anime series that Wallace lost that title to Steven later on. Wallace originally won the title of Champion in the manga, but declined the position in order to succeed his master Juan as the Sootopolis Gym Leader. He also made this decision in order to stay closer to Winona, whom he was involved in a relationship with. At this time, the Champion title was passed onto Steven. Eventually, Steven reclaimed his title as Champion after Steven returned to him the champion cape. It is assumed that Wallace returned to being the Sootopolis City Gym Leader in the Omega Ruby/Alpha Sapphire manga special.
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