Saizou is a man from Kazuki Fuuchouin's past being a childhood friend who was studying at the Fuuchouin School with him before its fall. It's noted that Saizou's clan was also killed by the Ura-Fuuchouin, thus making his alliances with both Fuuga and the Kokuchouins highly suspect. Along with the training he learned from Fuuchouin String Style, Saizou has also managed to master the Black Threads techniques. Later, it's revealed that he was forced to join Yohan as the young scion implanted a "dark cocoon" made of black thread in his heart (This is also why Saizou did not look up Kazuki and the rest for so long, as he knew that he HAD to betray them to save himself.). His family school focuses on "strength" and his ultimate skill involves generating fire from the friction of his threads. However, that is still not a match for Kazuki, whose ultimate is to generate a vacuum from his spinning threads (much like Yukihiko Miroku's "Infinite Moon"). After losing to Kazuki and explaining his plight (which includes the revelation that the members of Fuuga are trapped in the cocoon), Saizou tore his heart out, which liberated the captives.
Past and "Betrayal"
Saizou used to train under the Higashi Fuuchouin and was to become its next leader. In a duelling tournament between the Higashi Fuuchouin and the Omote Fuuchouin, where Saizou emerged the victor, he asked as his prize a battle with the son of the patriarch, the lord Kazuki Fuuchouin . Kazuki's fame as a talented child in the string arts had since spread to every other Fuuchouin school, albeit Saizou had never seen his face. Upon a call from the patriarch, Kazuki, whom Saizou thought was a girl enjoying his antics over the duel contests, emerged beaming and introducing himself. Battle and conviviality are two different matters, though; Kazuki lashed out at Saizou, destroying stone pillars and, almost, Saizou's face, in the wake of his strings. The battle between the two heirs ended in a draw which, Saizou admitted himself years later during their re-match in the Infinity Fortress, was just a stroke of luck for him.
Saizou is the heir to the Higashi Fuuchouin, which also uses strings as the Omote Fuuchouin. On the other hand, the Higashi Fuuchouin-ryuu emphasizes strength, and uses feathers to store the strings. Kazuki comments that the movements of Saizou is such that he resembles a feather fluttering in the air, suggesting speed and agility.
Saizou is a tragic character. As a talented youth he managed to defeat hundred seasoned Kokuchouin fighters alone during the Omote-Ura war. However he was caught by Yohan and brought to the Infinity Fortress, where subsequent events led to the establishment of Fuuga. Throughout the years of Fuuga he lived in fear that he will eventually betray Kazuki because of the existence of the 'black cocoon' seed inside him. He wished to commit suicide, but was stopped by Yohan, who proposed a plan instead to allow his friends and Kazuki to live yet a little longer. He submitted to the plan (which he frequently calls 'the script to escape all this chaos', referring to the ascend into Babylon) and disappeared from Fuuga.
He reappears after Kazuki summons him with the Red String. During the Queen's Cup Challenge, he unveils himself as allied to the Ura, and proceeded to take away Juubei Kakei, Toshiki Uryuu and Sakura Kakei, whom he branded with the Black Seal. In 'Get Back the Lost Time', Saizou is frequently probed by Kyoji Kagami, who said he has potential to rival Yohan, but must have a reason to let himself be allied to him (as it turned out, this reason is to protect Kazuki). Eventually, he leads the fallen Fuuga to attack the GB duo, who is nearing the entrance to the Beltine. He fights against Haruki Emishi, who is defeated utterly, before Kazuki turns up. During the battle between the Fuuchouin heirs, Kazuki constantly tells Saizou his betrayal is a wrong thing, but Saizou keeps his silence. As it turns out, even the Black Seal activation cannot defeat Kazuki's 'Empty Moon', and Sakura's conscience tells the true story to Kazuki, saying that Fuuga still lives (i.e. Saizou's betrayal is not as it seems). Kazuki is reluctant to pluck out Saizou's heart, but Saizou, smiling and joking how the Getbackers has 'ruined all his little plans', rips out his heart and offered it to Kazuki, saying 'farewell, beloved Prince of Fear' (the title is actually given by Saizou to Kazuki upon the latter's receival of the 'Stigma': 'people will fear your gentleness and your strength, and they will call you the Prince of Fear'). Eventually he died in Kazuki's arms. Upon his death, Kazuki draped the flag of Fuuga over Saizou, and forgave his deeds.
* Saizou is yet another character who had a childhood crush on Kazuki's youthful feminine escapades. When he died, Saizou gave up his heart to Kazuki (literally) saying 'farewell, beloved Prince of Fear'.