From Bulbapedia, the community-driven Pokémon encyclopedia.
Mary (Japanese: メリー Merī) is a member of the Pokémon Mystery Club. Along with her partner Ken, they try to solve Pokémon mysteries. Their main mission is to prove that Pokémon came from space. Her Japanese voice actresses are Ochiai Rumi (EP246) and Shindo Naomi (AG087).
In their first appearance, Mary and Ken were trying to capture a Cleffa that had fallen from a spacecraft. They wanted to see where it had come from. Ash and his friends, not wanting Cleffa to be harmed, tried to stop them from taking it. Ash sent his Phanpy to protect Cleffa. The young Pokémon successfully brought Cleffa home to its spacecraft and away from Mary's grasp.
When Mary and Ken reappeared in Advanced Generation, they were up to their old tricks again. This time, they wanted to capture a Lunatone. Just like last time, they were sent blasting off.