Serena.

Serena debuts in XY001 reluctantly leaving home after being pressured by her mother Grace, a former Rhyhorn racer, to follow in her footsteps. The Rhyhorn-racing path doesn't suit her; she'd met Ash years before at Professor Oak's Pokémon Summer Camp in Pallet Town (a story the show treats as the explanation for her crush). She picks Fennekin from Professor Sycamore. Her arc is finding her own path independent of her mother's expectations: she discovers Pokémon Performer culture (Kalos's answer to Contests, more dance-and-fashion-coded), formally enters the Pokémon Showcase circuit, and competes for the title of Kalos Queen.

Her rivals on the Performer track are Shauna, Miette, Nini, and reigning Kalos Queen Aria. Reaches the Master Class of the Pokémon Showcase but loses to Aria in the final. Cuts her hair in XY060 to mark a fresh start (the show treats it as a major character beat). Departs in XY141 to enter Hoenn Showcases. The XY141 airport scene contains the only canonical romantic gesture in the entire Ash run: Serena kisses Ash goodbye on the cheek. The fandom called it Amourshipping and never quite recovered.