Elder Princess Shroob

"I LAUGH AT YOU. PATHETIC MUSHROOM HEADS. YOU GATHERED THE COBALT STAR SHARDS WITHOUT EVER GUESSING THE TRUTH!~Elder Princess Shroob

The Elder Princess Shroob is the elder sister of Princess Shroob, and the true final boss of Mario and Luigi: Partners in Time.

Vs. Peach
Princess Peach enters a time machine designed by professor E. Gadd and ventures into the past- at the same time The Shroobs captured the Mushroom Castle of the past. She is then faced by both Princess Shroobs. In desperation, she grabs the time machine's power source, the cobalt star, and manages to trap the Elder Princess Shroob's essence within it. Peach is then captured by the younger Princess Shroob.

The Cobalt Star
During the events of the game, Mario and Luigibelieve that they should gather the shards of the cobalt star to be able to rescue Peach. The Princess Shroob contacts the Mario Bros. and pretends to be the spirit that resides within the star, telling them that if they gathered the shards, then they could defeat the shroobs.

Battle and Defeat
After the Mario Bros. kill the younger Princess Shroob, young Bowser drops into the room, steals the cobalt star, and puts it together, which releases the Elder Princess Shroob. Mario and Luigi defeat the Elder Princess Shroob with some assistance from Peach. But then, she taps into the power in her crown, and mutates, showing her true form, and knocking Peach unconscious. Mario and Luigi then have to defeat her by attacking her crown, which made her nearly invincible, then attacking the Elder Princess Shroob herself. After being beaten, she swears that the Shroobs will have vengeance, and turns into a small purple mushroom, which Baby Luigi picks up.

Shrowser
When The Mario Bros. arrive at the Mushroom Kingdom of the present, they find Bowser unconscious. The mushroom that was once the Elder Princess Shroob floats out of Baby Luigi's pocket, and into Bowser's mouth, turning him into Shrowser.