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After the bountiful feast, Frank and the Great Ancestor moved into the shrine of the Monk Clan, where they completed their engagement while holding hands, there wasn't any need for any kiss or something.
The shrine of Amaterasu suddenly released bright sunlight over the pair, as if they were being bathed by the grace of Amaterasu herself. Through it, she "recognized" their engagement and transferred every perm to Frank, which also included the ability to use the millenary artifacts, scrolls, and more!
Of course, if he had not been recognized, the artifacts and other items would have never responded to him and could have even self-destructed.
"Can you tell me your name now? Calling you Great Ancestor is…" said Frank.
"Ah! I am deeply sorry for never telling you my actual name, my lord… My name is Kaguya Amaterasu," said the Great Ancestor.
"Kaguya… That's a pretty name," recognized Frank.
Check "The Protagnist's Sister Is Actually the Strongest"!