From Bulbapedia, the community-driven Pokémon encyclopedia.
A Frontier Brain (Japanese: フロンティアブレーン) is either any one of the Pokémon Trainers who preside over a Battle Frontier facility, or one of the top performers in an online Pokémon contest held by Nintendo in early 2005. The Frontier Brains in Emerald, Platinum, HeartGold and SoulSilver are the former. Those from Hoenn hand out Frontier Symbols to those Trainers who rise through the ranks and show true skill in each of the facilities, while those from Sinnoh and Johto hand out commemorative prints for the same reason.
Hoenn Frontier Brains
Sinnoh/Johto Frontier Brains