From Bulbapedia, the community-driven Pokémon encyclopedia.
Pokémon Mystery Club (Japanese: ポケモンフシギクラブ Pokemon Fushigi Club) is a duo of two overzealous investigators: Ken (ケン Ken) and Mary (メリー Merī). They first appeared in EP246 and later reappeared in AG087. Like Team Rocket, they have a motto, but consider themselves to be heroes despite their dubious methods. Although they don't use Pokémon of their own, they have a variety of scientically advanced equipment and gadgets including bulldozers, jetpacks, robots, and rocket launchers which they build themselves using tools from the bag that Mary carries. They usually call themselves 'PMC' for short.
In the Japanese version, Mary was voiced by Ochiai Rumi in her first episode and Shindo Naomi in her second. Ken is voiced by Seki Toshihiko in both.
Their goal seems to be to do whatever it takes to conclusively prove that Pokémon are extraterrestrial lifeforms, which causes them to be motivated into capturing and stealing Pokémon species which have relations to outer space, such as Cleffa and Lunatone.