Subaru is one of the Steel Saints Mitsumasa Kido's technologically enhanced special backup unit for Athena's Saints, created decades ago.

One day, after learning about Pegasus Koga and his deeds, climbing the Twelve Temples and defeating both Mars and Abzu to be specific, he appeared at a Steel Saint training facility. He trained harder than anyone, never speaking to anyone, until one day he met Erna, another Steel Saint. He spoke to his new friend, declaring his intention to become like Koga, and to defeat him.