Olivia

Olivia is Mario's main ally in Paper Mario: The Origami King, taking a similar role to Kersti in Paper Mario: Sticker Star and Huey in Paper Mario: Color Splash. She is King Olly's younger sister, although it was later revealed that he essentially created her. Her traits include her folded crown and hair, her yellow robe, and her spiky limbs. She can transform into a Vellumental on certain Magic Circles. This can be done on the field or in battles.

Background
Olivia was created by her elder brother King Olly as part of his plan to rule over the Mushroom Kingdom; sitting in the throne beside his to act as the queen to his king. However, Olivia rebelled against Olly's plan, viewing it as her duty to stop him. King Olly punished Olivia by trapping her in a wall in Peach's Castle.

Paper Mario: The Origami King
When Mario explores the dungeon in Peach's Castle, Olivia calls out to him, saying she is stuck in an "in-between dimension". Mario uses the 1,000-Fold Arms to free Olivia sandwiched in between the wall sheets. Olivia travels with Mario on his journey, explaining the battle system and how to fill in Not-Bottomless Holes. Olivia was responsible for naming Bob-omb "Bobby", and enables Mario to use Magic Circles. Olivia is usually very cheerful, but when Bobby explodes to save her from a boulder in Sweetpaper Valley she becomes uncharacteristically sad. Olivia only cheers up when Mario uses a Goomba Mask to make a funny face, making her realize that Bobby wouldn't want her to be sad. She can also take on the appearances of the Vellumentals, and transforms into a giant hammer for Mario to use in the final fight with King Olly.

After King Olly died upon being beaten by Mario and Olivia, she, as part of King Olly's final request to make amends for the horrors he enacted, folded up his body to be the final crane for the Thousand Cranes technique, and wished to undo everything King Olly caused. As she had been created by King Olly, that also results in her own demise, though the 100% ending displays miniatures of her and her brother created by the Origami Craftsman.

Paper Mario: The Origami King

 * European website bio: "Olly's younger sister, Olivia uses her powers for good - teaming up with Mario to take down her older sibling. Olivia loves adventure, and will help you find the right path if you're ever stumped"

Trivia

 * In the Japanese version, Olivia tends to finish her sentences with "ですわ" (desu wa), a stereotypical speech pattern often used by princesses, noblewomen and arrogant women in Japanese media, similar to how Princess Peach talks in Itadaki Street DS and Fortune Street.