The sleepy little kingdom of Pontus in what is now Northern Turkey was a backwater in the Hellenistic era. Its rulers affected a Greek culture, but ruled over a largely Persian land, that had changed little since Alexander overthrew the Achaemenid Persian empire.
Mithridates V Euergetes married a princess named Laodice, from the Seleucid dynasty that ruled Syria and was descended from one of Alexander’s generals. She bore him two sons, and a gaggle of daughters. The elder son was Mithridates VI Eupator, known as the Great, who achieved fame as an enemy of Rome who had the military and political talent to defy even Sulla and Pompey. But what of his brother?
The literary sources are very limited. Strabo tells us:
Dorylaüs was a military expert and one of the friends of Mithridates Euergetes. … But when, a little later, he learned that Euergetes, as the result of a plot, had been treacherously slain in Sinopê by his closest associates, and heard that the succession had passed to his wife and young children, he despaired of the situation there and stayed on at Cnossus…. Now Euergetes had two sons, one of whom, Mithridates, surnamed Eupator, succeeded to the rule when he was eleven years old. Dorylaüs, the son of Philetaerus, was his foster brother; and Philetaerus was a brother of Dorylaüs the military expert. And when the king Mithridates reached manhood, he was so infatuated with the companionship of his foster brother Dorylaüs that he not only conferred upon him the greatest honours, but also cared for his kinsmen and summoned those who lived at Cnossus.
Appian tells us:
He [Eupator] was bloodthirsty and cruel to all – the slayer of his mother, his brother, three sons, and three daughters.
Photius includes an epitome of Memnon:
After this, the grievous war between the Romans and Mithridates king of Pontus broke out; the apparent cause of this war was the seizure of Cappadocia. Mithridates gained control of Cappadocia when he captured his nephew Arathes after breaking his oath concerning a truce, and then killed him with own hands. This Arathes was the son of Ariarathes and of the sister of Mithridates. Mithridates was a persistent murderer since his childhood. He had become king at the age of 13 years, and soon afterwards he imprisoned his mother, whom his father had left as joint ruler with him, and eventually put an end to her by violence; he also killed his brother. 
No literary source records the brother’s name. Fortunately there are two inscriptions from Delos that do mention it, which are accessible:
[βα]σιλέω[ς Μ]ιθραδάτου Εὐπάτο[ρ]ος [Ε]ὐ[— — —]
[καὶ το]ῦ ἀ[δελφοῦ α]ὐ̣τοῦ Μιθ[ρ]αδάτο[υ]
[Χρ]ήστου Δ[ιονύ]σιος Νέωνος Ἀθ[ηναῖος]
Διὶ Οὐρίωι ὑπὲρ βασ[ιλέως]
καὶ τοῦ ἀδελφοῦ αὐτοῦ
καὶ τῶν πραγμάτων αὐτῶν.
To Zeus Ourios on behalf of King
and his brother
and their fortunes.
His name, then, was Mithradates Chrestos or Chrestus. He suffered the fate that rivals to the throne traditionally suffered in oriental despotisms — to be murdered by a successful sibling.
Is anything else actually known about this man, or boy? I can’t find any other sources that mention him.
-  Strabo, Geographica, Book 10, chapter 4. Online here. ↩
-  Appian, History of Rome, c.112. Online here. ↩
-  Photius, Bibliotheca, codex 224. Online here. ↩
-  B. C. McGing, The foreign policy of Mithridates VI Eupator, King of Pontus, Brill, 1986, p.89, accessible sometimes in preview. ↩
-  Ins. Delos 1560 (Durrbach, Choix D’Inscriptions de Delos 187, no. 113), ca. 115-4 BC. Accessible online here. ↩
-  IDelos 1561, ca. 121-111 BC. ↩
-  A. B. Cook, Zeus: a study in ancient religion, CUP, 1914, p.154. Preview online here. ↩
-  http://www.ancientlibrary.com/smith-bio/2204.html ↩