Well, the whole setting is a fantasy (maybe a blend of comedic and urban fantasy is a decent description of this) world, so these names probably should be fine there.
Aand the relation to the two's personalities is only my own little sighting, because I first chose the names and then started to work on the other aspects of the characters without being inspired by their names' meanings at all.
Maybe I should've said that A) the brothers have magical powers: ice and water, respectively, and B) there is a water theme not only in their own names, but in the name of their father and other relatives from his side (maybe this family is actually a 'water clan' like Higuma said), so it's a tradition, too.
About the surname: yes, it is perfectly real. I always do some research before naming something or someone.
https://en.m.wikipedia.org/wiki/Satoshi_Hiyamizuhttps://m.imdb.com/name/nm2522991/Yeah, Japanese people usually do not have middle names, thank you for the small reminder.
And writing the surname first is just a usual thing for me (and the Japanese, too), heh.
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