@Kaarme @Thiron
I mean all of his classmate use his given name, i would imagine he simply introduced himself with that due to not liking or being uncomfortable with his family name.
Especially considering that he did marry into this family if I understand correctly (I dont know if you take the name in the land of the rising sun).
Kamikizaki also might sound slightly obnoxious? Again my incredibly limited knowledge of the language sees somthing about divinity, so perhaps it's an old family with reputations to uphold and hes not allowed to use it in vain.
In the household it would be plain old wierd to use the family name as it would get super confusing.
And if it's just like classmates know each others full names... nah... I worked with the same people for the last decade and i only know my managers surname and another because of "BRING THE PAYNE!"
I mean theres loads of reasons why she might not including her being a bit of a div.