Hephaestion son of Amyntor, a Macedonian aristocrat, is known in history for being Alexander the Great's closest friend, second in command. https://en.wikipedia.org/wiki/Hephaestion
Not according to Chambers Biographical Dictionary. According to them, it was Perdiccas (d.321bc.) :"He was second-in-command to Alexander the Great. He was virtually regent of the empire after Alexander's death, but was soon murdered by mutineers from his own army." (p.1446)