This seems interesting; I actually have a couple of characters, if you don't mind.
Name: Jett Dellis
Age: 16
Role: Friend to Drake
Appearance: Jett is an above average height for his age, and is fair-skinned. His hair is a deep jet-black (surprise surprise), and his eyes are a violent shade of navy blue. He wears an intensely-designed navy shirt with a Heracross design, and he wears an unzipped vest over it with red stripes. There used to be sleeves, but he had them ripped off so he could move his arms easier. He also wears grey jeans tha have been worn, but not ripped. He wears a pair of high-quality running shoes, and he has a one-strapped black and blue backpack. He also has a belt, on which he keeps his team.
Personality: Jett isn't your run-of-the-mill rival; he isn't overly confident, cocky, and selfish, yet he isn't way too nice. He's that gray spot in the middle. Jett is fierce, and a force to be reckoned with in battle. He pushes himself and his team to the limit and beyond, but is caring about his allies and respects them, and recognizes their weaknesses. Jett met his partner when he was young, and they grew together, both taking on a more serious demeanor. He'll be nice to those around him, but might throw in a friendly insult here and there, or give helpful criticism to his rivals and opponents.
How they should interact: I was thinking Jett would be that rival who isn't beaten until the very end, all of the battles before ending in either losses or ties. He'll notice a battle that Drake has, and will challenge him after; or, he could defeat one of Drake friends in a battle, asking if Drake wants to trade blows next. He'll keep one-upping his rival, sometimes assisting him in Tag Battles and helping out with the evil team, eventually meeting Drake at the Pokemon League, saying something like, "Whoever wins this gets to go in there first." By then his partner will (hopefully) achieved Mega Evolution. Once the battle ends, Jett will probably give Drake something useful for the League, then walk back to Victory Road to train more.
Pokemon:
Heracross (Partner and usually ace Pokemon)
Eevee (soon to evolve to Umbreon)
The rest of his Pokemon he won't catch until later on.
Zorua (soon to Evolve
Mimikyu
Riolu (soon to Evolve)
Haunter-Gengar
Name: Cobalt Feiron
Age: 26
Role: 8th Gym Leader (or 7th, if 8th doesn't work for you)
Appearance: Cobalt is tall, with silvery dark hair. His eyes are a deep blue, and he wears a gray sweatshirt with the Gym logo on it. He resembles a blacksmith, with a rugged, very muscular look. He has fingerless gloves, and several burn scars on his arms. He wears a blacksmithing apron on his back, but tied so it flows like a cape. He also has a pair of deep, dark brown combat boots. He has a strap of leather that goes around his shoulder with Pokeballs hanging from the belt.
Personality: Cobalt focuses on the tiny details, battling with an unbreakable seriousness and teacher-like demeanor. When an opponent is doing badly, he offers criticism on how he's getting the best of them. He battles to bring out the best in his opponents, so he'll feel like they both gave it their all when offering his challengers the Forge Badge.
How they should interact: Again, Cobalt is the 7th or eighth Gym leader. His Gym is like a forge, the arena being a domed warehouse-sized area in the back with blades mounted on the walls, and massive fires burning in the four corners of the room.
Pokemon:
Escavelier
Bisharp
Aegislash
Magnezone
Scizor (Mega, Ace)