Eumenes was a Thracian, according to Plutarch's The Life of Eumenes:
Duris reports that Eumenes, the Cardian, was the son of a poor wagoner in the Thracian Chersonesus, yet liberally educated, both as a scholar and a soldier; and that while he was but young, Philip, passing through Cardia, diverted himself with a sight of the wrestling matches and other exercises of the youth of that place, among whom Eumenes performing with success, and showing signs of intelligence and bravery, Philip was so pleased with him as to take him into his service. But they seem to speak more probably who tell us that Philip advanced Eumenes for the friendship he bore to his father, whose guest he had sometime been. After the death of Philip, he continued in the service of Alexander, with the title of his principal secretary, but in as great favour as the most intimate of his familiars, being esteemed as wise and faithful as any person about him, so that he went with troops under his immediate command as general in the expedition against India, and succeeded to the post of Perdiccas, when Perdiccas was advanced to that of Hephaestion, then newly deceased. And therefore, after the death of Alexander, when Neoptolemus, who had been captain of his life-guard, said that he had followed Alexander with shield and spear, but Eumenes only with pen and paper, the Macedonians laughed at him, as knowing very well that, besides other marks of favour, the king had done him the honour to make him a kind of kinsman to himself by marriage. For Alexander's first mistress in Asia, by whom he had his son Hercules, was Barsine the daughter of Artabazus; and in the distribution of the Persian ladies amongst his captains, Alexander gave Apame, one of his sisters, to Ptolemy, and another, also called Barsine, to Eumenes.
The Thracians are a distinct tribe from the Scythians, so that part of Hitoshi Iwaaki's retelling is historically inaccurate. That said, both tribes were consider barbarians (~ non Greeks) in classical Greece, so that part is accurate.