From the Super Mario Wiki
Twink is a Star Kid in Paper Mario. He is a star that helps Princess Peach in her time of need when she gets kidnapped by Bowser. He frequently gives Mario and his partners information on where the Star Spirits are being held captive. He was born in Starborn Valley. He was then ascended to Star Haven immediately after Bowser stole the Star Rod. If he had ascended any earlier, he would have been trapped there and unable to assist Peach throughout the course of the game. Some dialogue from the other Star Kids still in Starborn Valley suggests that he had somewhat of a big brother relationship with the others and looked after them when he still lived there.
When Peach was first captured, she wished for help, and Twink arrived. Since he was so young, he wasn't able to grant any big wishes. He could, however, assist Peach in eavesdropping around her castle.
At first, Peach asked him to carry Lucky Star to Mario and tell him that she was fine. Twink recognized Lucky Star. After Mario visited Shooting Star Summit, Twink knocked into him on his way back to Toad Town. He kept saying sorry but turned happy recognizing Mario. He gave Mario Lucky Star, and asked him if he wanted an Action Command lesson. Then he told Mario that Peach was fine and left. Goombario spoke saying he was brave.
After Chapter 1, Twink and Peach found Bowser's Diary which revealed the second Star Spirit's location. Princess Peach asked Twink if he could tell Mario about it, and Twink said he was willing to do so. When Mario returned to Toad Town, Twink greeted him, praised his work on defeating Koopa Bros., and told him about Mamar's location.
After Chapter 2, Twink was hiding behind a wall when Bowser was talking to Peach. He heard Bowser talking about the "Invincible" Tubba Blubba, and suggests searching for the information. They snuck out and heard that Tubba Blubba had a weak point, but Mario defeated Tubba Blubba before he figured out what it was.
After Chapter 3, when they were trying to sneak out, Twink heard from Bowser that Mario's next destination would be Shy Guy's Toy Box. When Mario went to the west of Toad Town, Twink found him, praised his work on defeating Tubba Blubba, and told him about Shy Guy's Toy Box.
After Chapter 4, when Peach was going to cook something for Gourmet Guy, they came to the kitchen in the castle. Twink picked a cookbook in the kitchen, and suggests "Special Strawberry Cake". Peach agreed and Twink read the instructions for Peach. Gourmet Guy enjoyed the cake and told Peach about the fifth Star Spirit's location. Princess Peach asked Twink if he could tell Mario about it, and Twink said he was willing to do so. After Mario rescued Muskular, Twink came and, seeing Muskular, called him "the honorable Star Spirit". Muskular recognized Twink and he said he was touched. After Muskular went up to the sky, Twink told him about Misstar's location.
After Chapter 5, when Peach was discovered by Mr. Hammer, Twink was frightened and hid behind a vase. When Peach was taken to 64th Trivia Quiz-Off, he became happy when Peach answered a quiz right, and sad when Peach answered wrong.
After Chapter 6, Twink simply followed Peach.
After Chapter 7, after the Koopatrols tied Peach up, Twink tried to attack the Koopatrols but was knocked out of the castle.
When Mario and his partners fight Bowser, Twink decides to take on Kammy Koopa. When Twink is fighting, he can only use Dash, while Peach can increase his Attack Power with her wishes (expressed as Focus). Once Kammy is defeated, Mario is given the upgraded version of the Star Beam, the Peach Beam, in which Twink assists the other Star Spirits, making it possible for Mario to defeat Bowser.
At the end of the game, Twink bids farewell to the princess, telling her he's going to make people's wishes come true. It seems that Twink wanted to tell the Princess something further, but he stopped himself and rose up to Star Haven.
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