One day, Sakura opened a strange book in her father's library that contained magical cards. Not knowing their power, she took the first one and read out loud the word "Wind".  The card shined and a gust of wind came from it, scattering all the cards everywhere out of her house. Because of that accident, she had the obligation to find them all back or disaster would strike the world.

Soon after the accident, at school, there was a new boy from Hong Kong in her class. He had been sent from his family's clan to find the cards too because he was a descendant of Clow Read, the magician who created the Clow cards. The first time they actually met was in the schoolyard where Li threatened Sakura and asked her to give him the cards she had found so far. Sakura refused and they became rivals until Li realized that Sakura was strong and worth of his trust.

They then fought side by side gaining all the Clow cards back. Sakura made him discover how important friends were for Li was a solitary person. He came from Hong Kong with only one goal and didn't care about anything else. He used to keep his feelings away from his mind, concentrating only on his mission. Sakura being such a cheerful and naive person, she picked Li's curiosity. Even though Sakura was strong, she was also vulnerable, and Li knew it from the beginning. It came up very slowly, but Li decided to protect her at any cost.

Their friendship became affection and it is obvious that Li was the first to fall in love with her. Each time Sakura smiled at him or told him a compliment, he became as red as a tomato. But Sakura had someone else on her mind, her brother's friend Yukito. Too shy and knowing Sakura's feelings for Yukito, Li kept his feelings for himself.

Sakura and Li finally collected all the cards and they were ready for the final judgment which only Sakura passed making her the true mistress of the Clow cards. Li had the opportunity to go back home, now that he failed his mission. Instead, he decided to stay, hoping to be with Sakura a little more... and maybe one day... Sakura finally confessed her feelings for Yukito but he didn't returned them. Sakura was very sad and Li comforted her and told her that he was sure that she would find that special person someday, hoping secretly that he would be that someone.

They became closer, Li being always there for her. One day, he finally had the courage to tell her. Since she considered him as a dear "friend", she was stunned. Li then left, telling her to be careful going home. Some days went by with Sakura questioning herself constantly. Then she had a phone call from her best friend Tomoyo, telling her that Li was leaving for Hong Kong. The fact that she would never see him again made her cry and she knew that she loved him. She couldn't stand being without him in her life. That's when she ran to the train to tell him how she felt. (Quote above)

Syaoran must go back to Hong Kong now that Sakura's got all the cards. In the bus, Li's thinking about the moment when he confessed his feelings to Sakura. 

(Quote from the manga version.)  A Japanese legend tells that if you give the person you love a teddy bear and if that person gives your name to that bear, it means your love will live forever.

Li: [thought] "I didn't receive an answer... but..."

Sakura: "Li!"

Li: [opens the window] "Sakura!"

Sakura: [panting but still running]  "I...  [lifts the teddy bear]  I understand my feelings!  [hands the bear to Syaoran]  The one I love most is you!"

Li: [takes the bear]  "Thank you."  [smiles]  "Can I name this bear 'Sakura'?"

Sakura: "And can I name the one you gave me 'Syaoran'?"

Li: "Yes."  [The bus starts moving again]  "I'll come back."

Sakura: "When? When's that?"

Li: "It'll take time. Will you wait for me?"

Sakura: "Yes!"

[The bus is now far away.]

Sakura: "I'll wait forever because Syaoran is the one I love most"

Manga version:  Well, in the manga Li comes back a few years later, they are now young adults. He still has the teddy bear Sakura gave him. He tells her that he is now back for good and that they don't have to go through phone calls and letters anymore. Sakura jumps in his arms telling him that they'll now be together forever.

Anime version:  In the second CCS movie, Sakura still hasn't told him how she felt. She will eventually try but every time something happens. In this movie, Sakura has to capture the Emptiness card, but doing so, she learns that she'll have to sacrifice her most precious feeling: Li's love. If she lets that card free, the world will become emptiness itself so she has no choice, even if it breaks her heart it's the price to pay to save the world. She succeeds into capturing it and the card is transformed into the Hope card. Sakura sees Li and starts crying because she thinks he has forgotten her. She tells him that she loves him more than anything. Surprisingly, Li returns his feelings for her: their love was too strong to be broken!