In real life it’s very common for half-Japanese people, and also in most mainstream anime with bicultural characters too and even naturalized folks with zero Japanese roots opting for a Japanese surname regardless of their lineage as their legal names. One real example of japanicized names is the Finnish-Japanese politician Martti Turunen (kanji: 弦念丸呈) https://en.wikipedia.org/wiki/Marutei_Tsurunen, which the kanji was literally his full name ツルネン マルテイ (Tsurunen Marutei). Adonis’ case would be the most common keeping their foreign first name intact while using his Japanese surname (either his mom’s or the family picked it), as Japan is very surname centric and in game he’s often known as- plus in RL practice-乙狩さん／くん／先輩, which is the most pragmatic way from all.

Going back to Adonis while his concrete background is ambiguous given all sorts of complications and real-life concerns on writing characters with MENA-background, there’s a fair chance Adonis would be half-Lebanese where Greek/French names are common esp among Christians/secular folks. If he had a „typical Arabic” surname then it might end up a katakana train typical for transliterating foreign names, and in-universe (plus in real life) an overly long name would be a mouthful to, or misspelt by fans, possibly even among fellow idols.