"Ryuji" was constructed following the Naming Conventions claim that "For the most part, [male Raen names] have a strong link to things in nature which are believed to be strong, agile, or even uncontrollable." I wanted it to be an actual existing name, and I liked contrasting imagery, so I ended up with 流 (Flow) and 地 (Earth), which together can be read as Ryuji. In backstory terms, this is a reading that Ryuji gave to his own name, the original being 龍二.*
As for "Hinoto": during the lead-up to Heavensward, it was suggested that old Japanese battleship names might make a good starting point for Raen surnames, and though there isn't one called the Hinoto, there is one called "Teibou" (丁卯). I was drawn in mostly by the meaning - it's an astrological term for part of the Chinese sexagenary cycle - but I also liked the sound of its alternate reading (Hinoto) as a surname. Using the Naming Conventions, I constructed a backstory of the Hinoto clan once being a part of a warlord's Sixty Armies, each of which was named after one part of the cycle; though he was defeated and his armies scattered, the Hinoto army was one that banded together as a clan and continues to exist as such in the present time.
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*"Dragon" and "two." Backstory stuff!
A fun bit of head-canon I adopted when writing Ryuji's backstory was that, in the times of antiquity when they first encountered and assimilated into Hingan/Doman culture, many Raen were given names that began with 龍 because of their appearance. Meanwhile, 二 - pronounced "ji" in this case - was a common suffix for second sons in the Edo period, where Raen names take influence from. Ryuji is the second son in his birth family, and his father was a traditionalist, hence 龍二. Some time after Ryuji was disowned by his birth family and adopted into the Hinoto clan as a ward, he selected the 流地 version himself. Incidentally, his original family name was Akinai (商), which means "trade" or "business" - this was a name selected by his previously-nameless father, a man who had risen to become a very wealthy merchant in pre-Garlean Othard and thus a part of the surname-having aristocracy, meant to evoke a sense of complete domination over his profession.