From Bulbapedia:
A Gym Leader (Japanese: ジムリーダー Gym Leader) is the highest ranking member of a Pokémon Gym. The main job of a Gym Leader is to test Trainers and their Pokémon so that they are strong and resourceful enough to compete against the Elite Four in the games or the Pokémon League in the anime. If a Trainer defeats a Gym Leader in battle, then the Trainer earns that Gym's Badge. Once a Trainer has earned 8 badges from a region, he or she is able to compete against that region's Elite Four or Pokémon League.
A Gym Leader's job is not just to test upcoming Trainers. They must oversee how their respective gym is run and to maintain it. Great care must be taken to make sure that their gym reflects the best environment for the Pokémon type and playing style of choice, usually an environment that suits that type of Pokémon used there (like pools in a Water-type gym or flowers and plants in a Grass-type gym). According to Morty, teaching young people how to battle with Pokémon can also be the job of a Gym Leader. In the games, Trainers must overcome various obstacles and challenges from other Trainers before they can challenge the Leader. Gym Leaders may also be involved in other activities when they are not battling other Trainers or training themselves, usually something to do with their town's particular attraction or landmark. Gym Leaders can also leave their gyms without notice (as Blue, Giovanni, Jasmine, and Wallace do), and even challenge other Gyms and the Elite Four (like Volkner wanted to do). In later games, it is made apparent that Gym Leaders know each other and are acquainted with Gym Leaders from other regions, and some travel to other regions: Jasmine travels to Sinnoh to take part in Super Contests, and is friends with Erika.
It has not exactly been made clear how Gym Leaders attain their position, but it appears that it varies for each gym. Some of them founded their gym and are self-appointed, others appear to be passed on through families, while a few are chosen by the other members of the region's Pokémon League. Leaders can also choose to vacate their position: reasons for this can vary. Blue is a former Champion appointed to be leader of Viridian Gym when Giovanni vacates the position, while in the anime Agatha takes Giovanni's place as a temporary leader. Koga was the leader of Fuchsia Gym until he was promoted to a member of the Elite Four; his daughter Janine replaces him as the leader. Wallace vacated his position in Sootopolis Gym when he became Champion, leaving his mentor Juan to take his place. Falkner's father is also mentioned to have been the leader of Violet Gym before Falkner took the position. Roark's father Byron is a Gym Leader like Roark himself, but the two operate entirely different Gyms within the same league.
Sinnoh is the only region where all the Gym Leaders are in cities, unlike previous generations where at least one is in a town.
In Pokémon Adventures, Red and Falkner have to battle with Pokémon set by the Pokémon League, where those Pokémon are not controlled by Trainers and the challenger is not allowed to let any of his Pokémon faint.
Although similar in nature, Frontier Brains are not considered Gym Leaders and are thus not included in this list.
Kroel Gym Leaders[]
Overall, there are 16 Gym Leaders in the Kroel Reigon. They are divided into two groups, North and South, each consisting of eight Gym Leaders. Note that none of the Gym Leaders share the same type or any of the same Powermon.
Northern Region[]
The eight Gym Leaders of the Northern Region were designed by Darksilvania. They are notable for being more formal than their Southern counterparts, and most have at least one personality aspect that makes them seem menacing.
| Name | Type | Badge | City | Main Powermon | Other Powermon | Theme |
| Pollock | Poison | Waste Badge | Cauldron City | Polustrial |
Vermingo Opesttum Epidenza Smokroach |
Industrial CEO |
| Artie | Ice | Artic Badge | Frostbite City | Polanoir | Hyemalaya Narvharil Elephoreal Glaciartle | Artic Explorer |
| Smith | Fire | Smithy Badge | Brimstone City | Py-Rex | Centagirow Erymantoar Octanoleo Benzegatol | Blacksmith |
| Ellie | Electric | Overcharge Badge | Boltstrike City | Smilorm | Ampereohm Electrium Courobolt Voltortle | Rebel Rocker |
| Amelia | Flying | Honor Badge | Jetstorm City | Penguealth | Staglaizer Cranigami Atrimawk Bucefaloon | Air Force Fighter Pilot |
| Necro | Ghost | Ghoul Badge | Memorial City | Judorpse | Zombestein Infargeist Zonvodou Natchop | Surgeon |
| Jack | Dragon | Nightwing Badge | Blackfall City | Consteladon | Wyferno Draizelisk Notredoyle Fanbbeloki | Dragon Master |
| D-S | Dark | Fiend Badge | Ebony City | Lucefidon | Atrocean Nigtmalije Geminus Arkamera | Darkness Warrior |
Southern Region[]
The eight Gym Leaders of the Southern Region were designed by FawkesTheSkarmory. They are more informal than their Northern counterparts, and most are usually more friendly. All of the Southern Gym Leaders, with the notable exception of Michael, are based off fictional characters outside of the Pokémon franchise.
| Name | Type | Badge | City | Main Powermon | Other Powermon | Theme |
| Hakoda | Water | Deepsea Badge | Piscis City | Suborca |
Hiposeilor Corivancer Dracuda Venotreme |
Deepsea Explorer |
| Tak | Grass | Shaman Badge | Pupanunu City | Totemage | Fungila Bamboon Bellustona Croconstro | Jungle Shaman |
| Michael | Psychic | Vision Badge | Motown City | Psikosiko | Amphigolb Dementis Caphydring Gryphortle | Pop Singer |
| Barnum | Rock | Jurassic Badge | Triassic City | Omakapi | Stegorch Sharkarus Raptouka Mossiloth | Paleontologist |
| Pebbles | Ground | Terrain Badge | Bedrock City | Ankrash | Ocellorkey Subterosco Incalperor Zeurino | Wilderness Defender |
| Parker | Bug | Antenna Badge | Chelicera City | Cavaliant | Ignisect Terrapede Plagust Momorotha | Entomologist |
| Juniper | Fighting | Harmony Badge | Orchid Bay City | Galludight | Primareal Licab Thundare Iguaneleon | Martial Artist |
| Robert | Steel | Justice Badge | Metroville City | Wolverusk | Donferrote Majarafant Mirridel Hydraghing | Superhero |