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Frank let Miriam walk back to the orphanage with Ryujin, who was very clingy with Miriam for some reason, probably due to the mystical connection she suddenly created with him as she battled with him.
The little dragon god had grown fond of her a lot more than Frank could had possibly expected, but he decided to leave him with her so he could protect her, and as some sort of experimentation.
Inside of Miriam's orphanage, nobody had monster pets except the nuns, and the kids were always seemingly abused by them, who were quite the assholes.
So Frank wanted to break the power balance in there and help Miriam and the kids have some more space to breathe from their abusive caretakers.
Miriam held the cute Ryujin over her shoulders, as she walked back to the orphanage.
Check "The Protagnist's Sister Is Actually the Strongest"!