Princess Shroob

Princess Shroob is the main villain of Mario & Luigi: Partners in Time. She is, obviously, the leader of the alien race of Shroobs, setting off a full invasion of the Mushroom Kingdom to make it their own. She and her vast army of Shroobs attacked the Mushroom Kingdom of the past. Princess Peach arrives at that point in time in Professor E. Gadd's time machine, and is immediately attacked by Princess Shroob. Princess Shroob's older and more powerful twin sister was also there, and Peach managed to seal her soul inside the Cobalt Star and shattered it into 6 pieces. The younger sister kidnapped Peach for that. Possibly because Peach held the sixth Cobalt Star Shard, the Shroobs kept Peach unharmed and in perfect condition, aside from attempting to feed her to Petey Piranha.

When Baby Mario, Baby Luigi, Mario, and Luigi rescue Peach in Gritzy Caves, she has a purple mushroom attached to her face. She is later revealed to be Princess Shroob who had the intention of taking over the present Mushroom Kingdom as well as the Mushroom Kingdom of the past.

The four Mario Bros. eventually make their way through a shroobified Peach's Castle and hijack a Shroob Battleship and engage the Shroob Princess in a dogfight. After blasting the Shroob Mothership out of the sky the Princess emerges out of it. She then goes into battle with the bros. on her battle throne which can shoot lasers and change into a giant walker robot, it also protected by a forcefield that the bros. need to shut down.

After beating the Princess she ominously warns that her sister will avenge her.