Fairy type, Mega Evolution, full 3D — Pokémon's biggest visual leap.
X, Y · Released 2013
Kalos is the Gen 6 region, and the generation that finally moved Pokémon into full 3D graphics on the Nintendo 3DS. X & Y introduced Mega Evolution (temporary in-battle transformations that push Pokémon past their normal stat totals), the Fairy type (retroactively applied to 22 existing Pokémon), character customisation, and Pokémon-Amie affection mechanics. Kalos is also the smallest new-species generation since Gen 2 — only 72 new Pokémon, as development effort shifted to 3D model creation.
Kalos is based on France — Lumiose City is Paris (the Prism Tower is literally the Eiffel Tower), Coumarine City is the French Riviera, and Parfum Palace is Versailles. The legendary Xerneas (life) and Yveltal (destruction) reference the 3,000-year war lore that underpins the Kalos storyline, itself inspired by French Revolution themes.
Mega Evolution was originally conceived as a mechanic for every single Pokémon, but Game Freak scaled it back to a curated list after playtesting showed universal Megas would break game balance. Gen 6 shipped with 28 Megas; Gen 7 added 20 more; no new Megas have been added since.
X and Y added the Fairy type — the first new type since Generation 2 — specifically to rein in Dragon, which had dominated the previous two generations. Fairy is immune to Dragon, resists Fighting, Bug and Dark, and is super-effective against all three. The effect on the competitive metagame was immediate and permanent.
Mega Evolution arrived here too: a temporary in-battle transformation limited to one Pokémon per team, granting a large stat boost and often a typing change. It gave older Pokémon such as Kangaskhan, Mawile and Gengar a route back to relevance, and several were strong enough to warrant bans.
Generation 6 also rebalanced the type chart quietly — Steel lost its resistance to Ghost and Dark, making both far more usable as attacking types.
Before Generation 6, a Dragon-type attacker with a strong STAB move was close to unanswerable outside of mirror matches and Ice coverage. Fairy ended that in a single stroke, and team building since has largely been organised around the Dragon, Fairy and Steel triangle.
Kalos itself added only 72 Pokémon, the smallest count in the series, but several matter: Greninja's Protean ability made it one of the most flexible attackers ever released, and Aegislash's Stance Change was strong enough to see it banned from multiple formats.
To rebalance the type chart against Dragon, which had dominated Generations 4 and 5. Fairy is immune to Dragon moves and hits Dragon, Fighting and Dark for super-effective damage. It is the only type added since Generation 2 and it reshaped competitive play immediately.
A Pokémon holding its species-specific Mega Stone can Mega Evolve once per battle, gaining a substantial stat boost and sometimes a new typing. It lasts until the battle ends and only one Pokémon per team may do it, which makes the team slot a real cost.
72 new Pokémon, numbered #650 Chespin through #721 Volcanion — the smallest addition of any generation, offset by Mega Evolution giving dozens of older Pokémon new forms.