In Golden, Naoto starts with Sukuna-Hikona. It involves into Yamato-Takeru upon maxing Naoto's Social Link and can further evolve into Yamato-Sumeragi during winter.
Yamato is an old word for Japan, Sumera denotes the Imperial line descended from Amaterasu, and Mikoto is an honorific.
The English localization shortened the name to Yamato-Sumeragi. Sumeragi is another possible reading of 皇, and also refers to the presence of the Imperial line.
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References
↑Page description: Page features a table with Yamato-Sumeragi's skill table in it. "5 (Data)", Persona 4 the Golden Complete Guide, Dengeki Playstation. Published by ASCII. 358. ISBN-13: 978-4048866637.