The Strength of the Heart is the 6th chapter in Tsubasa: Reservoir Chronicle.
The 'goggle' gang member introduces himself as Asagi Shougo. He expresses that he is very impressed with Syaoran's kudan, and goes in for another strike. Syaoran blocks his water attack with a wall of flame. Just then, the police arrive on the scene and the gang scatters, with Shougo promising Syaoran that he looks forward to meeting him again.
Just as suddenly as it appeared, the flame disappears into Syaoran's body. Fai and Kurogane ask him if that was a kudan, but Syaoran is just as confused as they are. Syaoran turns in concern to the boys he protected from Shougo's attack, but to his surprise, the other boy disappeared into the other's body -- it was a kudan too, after all!
They then realize that Mokona has been separated from them. Just as Syaoran is frantically searching for Mokona's whereabouts, they realize that all this time it has been surrounded by girls, who are all fascinated by it's cuteness. Unfortunately, it turns out that while everybody was distracted by the fight, Mokona had sensed that one of Sakura's feathers was nearby, but it has since disappeared.
The boy that Syaoran protected introduces himself as Saitou Masayoshi, and offers to do something in gratitude for Syaoran's protection. Enthusiastically, Mokona suggests going somewhere to eat! Masayoshi takes them to an okonomiyaki restaurant, as the food is the specialty of Hanshin. Fai queries him about the fights, so Masayoshi explains that gangs of kudan users are formed to fight for territory and to demonstrate which users are stronger. It appears that there are also the "good" teams, which protect against vandalism from the other groups (Masayoshi himself is a member of one of these good teams) and the "bad" teams, which just destroy for the sheer act of it. Masayoshi deeply admires Shougo for being one of the "special" kudan users, but then tells Syaoran that his kudan is a "special" type too. He goes on to explain the difference between the various classes of kudan; "special" kudan only 'haunt' those with a really strong heart.