Aizen spent much of his career in the 5th Division, eventually becoming its lieutenant under the former captain, Shinji Hirako. Aizen also gained followers in Gin Ichimaru and Kaname Tōsen. A century before the series' setting, after Kisuke Urahara created the Hōgyoku, an item that he believed creates Soul Reaper/Hollow hybrids, Aizen began to experiment using the Soul Reapers that became the Vizards. Among his test subjects was Shinji, who knew Aizen was up to no good but learned too late that Aizen used his mistrust against him. Having also framed Kisuke for the Vizards' creation and forced his exile, Aizen learned the Hōgyoku actually grants its user's desires if they have the will to carry it out. But as Kisuke took the Hōgyoku with him, Aizen bides his time to take the item when the time comes.