The discovery of neutrinoless double beta decay (0νββ) would shed light on the persistent puzzle surrounding the origin of neutrino mass and help explain the matter-dominated universe. KamLAND-Zen is one of the leading experiments searching for 0νββ. The first phase of the experiment, called KamLAND-Zen 400, set a world-leading limit on 0νββ lifetime. After the conclusion of KamLAND-Zen 400, a brand new mini-balloon with a larger volume and cleaner surface were instrumented to contain 745 kg of Xe136. Since Jan. 2019, KamLAND-Zen 800 has started data-taking and aims to improve on the previous 0νββ result. A detailed study of the backgrounds in this new data will be presented along with a state-of-the-art approach for classifying backgrounds using a new algorithm called KamNet. The rejection power of KamNet does not rely on coincidence tagging and scales with hardware updates. With the help of KamNet, the 0νββ sensitivity of KamLAND-Zen 800 is significantly enhanced to a new summit.