From Bulbapedia, the community-driven Pokémon encyclopedia.
Io (Japanese: イオ Io) is the main villain of Phantom Thief Pokémon 7. She is a ruthless woman who will not hesitate to eliminate whoever gets in her way even if it's one of her own subordinates. Despite being a member of Team Galactic, she is not loyal to them and plans to use the Pokémon Darkrai for her own gains.
Io makes her first appearance as a silhouette in Stolen Memories giving orders to Lily to stop Pokémon 7 stating that it will be easy for the girl.
She makes her first actual appearance in Into The Hands Of Evil?! where it was revealed that she was in fact, the one who kidnapped Lily 5 years prior to the beginning of the story. Her first action in her revelation is to attack and knock out the Galactic Number Two for his failing to defeat Lily and states that he is a worthless fool. She then reveals her grand plan; to capture Darkrai and have it cover the world in darkness, using its hypnosis abilities so that she can have everyone on the planet do her bidding.
Hiori attempts to stop her but her Porygon-Z is too powerful and Io easily defeats him, she then proceeds to capture Darkrai in a special Poké Ball. Her success is short lived however, as said Poké Ball begins to overload and explodes, releasing the captive Darkrai who then goes on a rampage. She attempts to try to stop Darkrai with her Porygon-Z despite Hiori's warnings and is put to sleep by Darkrai's Dark Void. It was later revealed that she and the other members were rescued by Rocco and his Gardevoir when the cave that they were in began collapsing. It can be assumed that she is in jail for her crimes.
Her name comes from Io, one of Jupiter's natural satellites.