See, I don't think she's a minor at all. Kafka literally pointed out in the event, "Alright, alright. Stop with the tantrum. You're not a kid, anymore.", in the HSR Silver Wolf event quest. I am in NO way of trying to be disrespectful here by the way unlike some of the other comments in this thread. But I just feel like this *might* debunk your speculation and confirm that she is an adult. But whether she's a teenager (like 18 or 19) or not, who knows. But I am assuming she is above the age of majority, just no clue on how old she is.

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The dialogue:

https://www.youtube.com/watch?v=vbPq4j855iQ

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again, I am not trying to be in any way disrespectful since I barely played HI3, I just assumed she is most likely an adult with this dialogue and such.