The Boy From Pallet Town: Ash Ketchum (Satoshi) from Kanto to Kalos.
Kanto: The Rookie. Ash starts as an excited but immature and naive 10 year old boy from Pallet Town. He’s not really much of a trainer as much as he is a kid trying to be a trainer, and he’s always one step behind everyone else, especially his rival Gary Oak. The hot-headedness and self-centered ego hinders his progress throughout his journey and he needs the guidance of Misty and Brock to help him out along the way. Despite his flaws, Ash does have a kind and selfless heart when it comes to the well-being of Pokémon, putting his life on the line before his Pokémon in times of danger. Overtime, he gets a better handling of battles, but still has a lot to learn. Especially when his lucky winning streak runs out, and his neglect of handling his disobedient Charizard ends up costing him the Indigo League.
Orange Islands: The Learner. Ash is still immature as he was at the beginning of this arc, but he ends up taking a few humility lessons and battles (such as Lorelei) to deflate his ego and make up for his loss in the Indigo League. His arc with Charizard highlights this and his battle with Drake gives him his first real full 6 vs 6 battle. But while he may become better in terms of training and battles, so has Gary Oak, whose victory over Ash ignites an old rivalry anew.
Johto: The Experienced. The early episodes highlight a contrast of power between him and those starting out in Johto such as Casey. The team he posses from the Kanto region could easily allow him to breeze through the Gyms without much trouble, but it has detrimental effects such as Charizard being unable to properly train to the next level with the lesser experienced Pokémon, and Ash relying on his old dependable Pokémon instead of training the new ones he caught. Ash is not exactly mature (still has a hot-temper and can be foolish) but he is experienced enough to handle Gym battles without pity badges and is able to handle the League battles a lot better than before, culminating into beating his old-time rival Gary in a full battle. But there are still trainers much more experienced than him such as Harrison, if only barely.