“'I do'. Say it now, K?”
Nastasia is a villain in the game, Super Paper Mario, and the main antagonist Count Bleck's cohort. She is never fought directly, however she has powerful hypnotic magic, which she uses to brainwash all of her victims into serving Count Bleck, many of whom are encountered in battle. She can even use it to force people into doing certain things, as seen when she made Peach marry Bowser. She also acts like Bleck's secretary, trying to keep things running according to schedule and relaying his orders, even going so far as to repeat them verbatim to give them emphasis. She is also very close to Bleck, having pledged her life to him after he saved her some time before the events of Super Paper Mario. Bleck trusted Nastasia enough to let her know the story about his identity as Lord Blumiere and his lost love, Timpani. Her name is Greek for "She who shall rise up again", possibly a reference to her only falling unconscious from a deadly attack, or the story with the bat and the man. It is also a pun on the word "nasty" and possibly "Anastasia" or "Natasha".
If one goes to Flopside and listens to Carson at the bar, he will tell a story called "Of Bats and Men" (the title of which is a parody of Of Mice and Men). The story talks about a man searching for a girl he loved who is missing. While he is searching, he discovers a bat in a trap and sets it free. Later that night, while the man is sleeping, a woman whom he has never seen appears to him. In the story, it is said that the bat transformed into that woman and pledged, out of love, eternal loyalty to the man. No one knows what happened to them. It is assumed that the man is Count Blumiere, the girl is Timpani, and the bat who transformed into the woman was Nastasia.
She seems to use her power by lifting her glasses, upon which a white glow flashes and the subject she wishes to hypnotize is instantly under her control. Therefore, it could be argued she has tremendous hypnotic power and wears glasses to control this power. It can be noticed that they are only seen open once. The player gets a glimpse of them when Nastasia is crying over Count Bleck and she rubs her eyes under her glasses. They are seen closed after being knocked out by Dimentio, and she quickly dons her glasses before opening her eyes when she wakes up. Notably, sometimes her power causes changes in a victim, such as hypnotised Koopa Troopas donning dark glasses and spikes around their neck or Luigi changing his clothing after turning into Mr. L, although there are no other examples of this.
Nastasia once asked Count Bleck if he wanted to stop before it was too late while he muses over whether Tippi could possibly be Timpani. He refused and offered to let her leave with his blessings. She, however, decided to stay, as she had sworn her life to him ever since he saved her. She seems to have feelings for Count Bleck but does not want to attempt at him, because he is already very troubled by his past love experience with Timpani. In the end she cries due to Bleck's disappearance.
After Count Bleck is defeated by the four heroes, Nastasia takes a supposedly fatal blow from Dimentio that was meant for Bleck. However, she is later seen alive and well. When she learns of what the Count did to stop The Void, she starts crying uncontrollably. However, everybody still feels that Bleck and Tippi are still alive somewhere. Mimi and O'Chunks suggest that they build that perfect world that the Count had told them they'd make after disposing of the previous worlds. Nastasia, regaining her composure, agrees. After that, the player may find her on the third floor of Flopside, where she comments on her feelings for the Count, and decides that if she could not learn to love again, she probably never could have measured up to Timpani anyway.
 Names in Other Languages