From Bulbapedia, the community-driven Pokémon encyclopedia.
A Lass (Japanese ミニスカート Mini Skirt) is a type of Pokémon trainer that first debuted in the Generation I games. They are depicted as average young girls in cute clothing resembling private school uniforms.
They use a variety of Pokémon.
Blue was based on the early character designs of the Lass characters.
Lasses are the female counterparts of Youngsters
In the TCG
In the TCG, the Lass character has their own card in the Base Set.