SD Gundam: Musha Maruden

The 13th installment in the Musha Gundam series.

The Fallen Darkness Army seeks to alter the course of history by exploiting the rift in space-time that has opened at the Gate of Judgment. Musha Maru, determined to thwart their ambitions, pursues them across time and space, emerging in present-day Osaka as the 21st century dawns. There, Musha Maru meets a young boy named Susumu and finds himself living a carefree life as “Mucha-maru”!?

(Source: Kodansha, translated)