Sumaru is a genin from Hoshigakure. He dreams of one day becoming the Hoshikage. Like the other ninja of Hoshigakure, Sumaru takes part in star training, a technique which allow incredible chakra control, at the expense of severe physical health damage. He appears to be one of their more prodigious practitioners, being able to perform some of the more advanced techniques of star training at such an early age. Sumaru is told that his parents, Natsuhi and Hotarubi, were killed in battle guarding the fallen star, when in reality, they stole it in an effort to save the villagers from the damaging effects of star training. As a last favour, they asked the Third Hoshikage to lie to Sumaru to protect their family name.

Sumaru remembers his mother, especially, through a lullaby she sang to him as a child about a red star called Natsuhiboshi, which is actually the planet Mars. The lullaby serves as his motivation.