Cardcaptor: The Movie 2

Eriol Hiiragizawa's house is demolished to make way for a new amusement park in Tomoeda, activating a Clow Card, The Nothing, hidden underneath the house. After the park is built, she hides in its clock tower and begins secretly stealing the other cards from their mistress Sakura Kinomoto. Sakura faces her own challenges, having the leading role in a play her school is putting on as part of Tomoeda's annual festival, and her own feelings towards her friend Syaoran Li, who confessed to her before returning to Hong Kong. Sakura and her best friend Tomoyo Daidoji visit the amusement park, where Sakura senses a magical aura. Running into the park, she bumps into Syaoran and Meiling Li who returned for a visit, though Tomoyo and Meiling planned it to get Sakura to confess to Syaoran.
Tomoyo and Meiling make a hasty departure, leaving Sakura to invite Syaoran to dinner. She tries to confess but she is repeatedly interrupted by the Clow Cards guarding Kero and then the arrival of her brother Toya and his friend Yukito Tsukishiro. The next couple of days involve Sakura and her friends rehearsing the play and Sakura repeatedly attempts to confess to Syaoran but is interrupted. They also notice that things in the city are disappearing including a bridge in the local park. Sakura, Syaoran, Tomoyo and Meiling spend a day at the amusement park, with Sakura and Syaoran ending up on a Ferris wheel together where Sakura tries to confess again. However, both witness one of the Clow Cards vanishing and chase its aura to a hall of mirrors where they encounter the Nothing who steals several more cards.
Sakura and Kero are contacted by Eriol from England, who explains the Nothing was created to counter the positive magic of the Clow Cards with her own negative magic. The Nothing was released due to Sakura changing the cards' power from Clow Reed's to her own, and every time the Nothing steals a card, her power erases part of Tomoeda. Eriol warns Sakura that when she seals the card, her greatest feeling at the time, namely her love for Syaoran, will be erased as a payment. Sakura informs Syaoran, but he concludes the sacrifice is their only option. Sakura runs off in tears but is consoled by Yue, the second guardian of the cards and Yukito's true form. During another rehearsal, the Nothing attacks the school, injuring Takashi Yamazaki who was to play the lead role opposite Sakura, so Syaoran steps in.
During the play, the Nothing's power spreads and erases many of Sakura's loved ones including Tomoyo, Meiling, and her family. Sakura, Syaoran, Kero, and Yue go to the amusement park and battle the Nothing who erases Kero and Yue. Syaoran attacks her on the Ferris wheel but is caught in her destructive spheres and vanishes. Sakura pursues the Nothing to the clock tower, where she is stripped of her last cards apart from an unnamed card she created with her own magic after Syaoran left for Hong Kong. Sakura learns the Nothing collected the cards so she would not be alone anymore, but Sakura promises that she will never be isolated again and seals the Nothing. However, the required toll instead comes from Syaoran who tells Sakura he will fall in love with her all over again.
The Nothing and the nameless card fuse into one, becoming the Hope Card as Sakura tearfully confesses to Syaoran. She is shocked when he replies that he feels the same, discovering that the fusion averted the toll. The Nothing's powers are then reversed, reviving Tomoeda and its inhabitants. Sakura springs across the reforming clock tower to Syaoran's arms, together at last.

Question: who explains the Nothing was created to counter the positive magic?