Vivian

Vivian is Mario's fifth party member in Paper Mario: The Thousand-Year Door. She is part of the three Shadow Sirens, and is the youngest Sister in the group. She is constantly mistreated by Beldam, resulting in Vivian beleiving that she is lesser than her. As the game progresses, she gains more confidence (challenging her Sisters in the Palace of Shadow Fight), but still harbors some respect for Beldam (after defeating Beldam, Marylin, and Doopliss again, her response is 'I'm Sorry, Sis...').

Vivian's first appearance is in Chapter 2, when the Shadow Sirens were sent by Grodus to ambush Mario. Grodus gives the three a picture, and they go to Boggly Woods. Vivian found Flurrie's necklace while going there. However, Beldam loses the picture and blames Vivian for it, taking away a necklace from her. When Mario passes by again, it is discovered that Beldam had the picture all along, but is still angry at Vivian. They then challenge Mario to a fight. In this fight, Vivian is the middle strongest of the sisters and uses magic. After they are defeated, they run away, leaving behind Flurrie's Necklace. Vivian hesitates and goes back to get it, but ends up leaving.

Later on, at the Creepy Steeple, Mario fights Doopliss, a Duplighost. He steals Mario's body and identity, leaving Mario as a shadow.

Meanwhile, Vivian and the Sirens have got an 'infallible' plan. They have a weapon that will surely wipe out Mario: the Superbombomb. However, it is lost (presumably by Beldam). Vivian is sentenced to search for it while Marilyn and Beldam take a nap. She starts to cry while looking for it.

When Mario returns to Twilight Town, he finds Vivian searching for the Superbombomb. Mario finds it for her, but it is broken. He can ask to fix it, but Vivian will claim it's far too broken, and she asks him what he is doing here. Mario tells her about Doopliss and Vivian agrees to help get his body back.

When Mario and Vivian finally reach Doopliss' room in Creepy Steeple again, they battle him along with all Mario's old partners, who think Doopliss is Mario. Vivian realizes that the one she was helping was Mario, and for a few turns, will not help you in battle. After two turns, she returns because she thinks Mario was nicer to her than any of her sisters. At the very end of Mario's adventure, Beldam and Marilyn end their villain activities and promise not to mistreat her again.

Outside of battle, she possesses the same ability Bow has in Paper Mario, (which she hides Mario by making him invisible) but hers works in a more interesting effect: instead of literally turning invisible, Vivian can pull Mario into their shadow, where they can hide. This allows them to get past objects that move over them, and avoid sight from enemies and their attacks. Vivian possesses the strongest magic of any partner in the game, and is often thought of as the best partner in the game thanks to her fiery powers and the ability to burn enemies.

Gender
There is a controversy about Vivian's true gender in the original Japanese version; some claim that she is a pretty boy (a bishounen). However, in the Japanese version, Beldam of the "Three Shadows Group" uses the word majotachi, which refers to a group of sorceresses, to refer to them. Words like her, she, and sisters are also used. Unlike most characters that undergo major changes during translation, Vivian maintains her (female) name in both the English and Japanese versions. Beldam often calls Vivian "Man", but this could be Beldam's way of insulting Vivian (in the English Version, this insult has been changed to vivian being called 'Plug-Ugly'). More telling is that the menu information describes Vivian as a male, though this also could be a joke showing that Mario (who is supposedly journaling the adventure) misheard that Vivan was male.

In a rough translation of the character description of Vivian:

"One of the shadow group, Vivian appears to be a girl but is really a boy."

Also in a rough translation of Goombella's Tattle of Vivian:

"That's Vivian. Of the shadow group, she's the youngest sister... er, brother."

Trivia

 * Francis in Super Paper Mario has a Vivian plush doll.