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Arslan/Daryoon is heavily implied. It's like Alexander the Great & Hephaestion with roles reversed.
The anime's character designer, Kamimura Sachiko, has done a series of BL illustrations featuring basically expies of Darun and Arslan. She denied any connection but it was pretty much a case of "well, just because one of them looks exactly like Darun and wears exactly the same outfit, and the other looks exactly like my version of adult Arslan, right down to the clothes, jewelry and weapon, doesn't mean it's them!"