In the anime, Mikuri (Wallace) first appeared in "Our Cup Runneth Over!" as a Contest Master in Sinnoh. After watching Ash and Dawn's practice match against each other, he asks Ash to enter the Wallace Cup with his Buizel. He also does a practice appeal in the episode with his Milotic for Ash and the others.

He makes further appearances in Staging a Heroes Welcome, Pruning a Passel of Pals!, and Strategy With a Smile! acting as the judge for the Wallace Cup. He also awarded Dawn the Aqua Ribbon following her win at the Contest.

It is mentioned that, like in the game, Wallace was once the Sootopolis City Gym Leader and later the Hoenn Champion. It was revealed in "Battling the Generation Gap!" that the Pokémon stylist Lila designed his outfit for Contests.

In the Pokémon Adventures manga, he is the Sootopolis Gym Leader, in addition to being a contest master as well as the protector of the Cave of Origin. Wallace also takes Ruby under his wing, much like how Winona did for Sapphire. Despite having aligned himself with Team Aqua, he has a strong sense of justice, and is the first of the aligned Gym Leaders to renounce his loyalty, convincing Winona of the Magmas to do the same.

It was revealed in Volume 21 that Wallace was the Hoenn League Champion who had entered the Hall of Fame. However, he renounced the title as he expressed his wish of following his mentor, Juan's, footstep of becoming a Gym Leader of his city. According to Steven, what Wallace did not mention to the director of the Pokémon Association is that there is a second reason for his renouncement: He was trying to stay with Winona, the Fortree Gym Leader.

He and Winona apparently had some sort of relationship in the past, speculated by most fans to have been romantic. Although it is largely unconfirmed whether Winona still has feelings for him, it is quite obvious that he still has feelings for her, based on the scene where the two say their goodbyes before they leave to fight Kyogre and Groudon in Volume 20.

Wallace regained the title of Champion after Steven had a premonition that he would die in the upcoming battle between Groudon and Kyogre.

Wallace usually travels on an "Air Car," which can travel on both the land and the air. He is also the only Hoenn Gym Leader who nicknamed his Pokémon.

(Source: Bulbapedia, edited)