Who is Coenus from The Campaigns of Alexander and what is their importance?

Coenus successfully argued against Alexander's desire to push on past his most eastern position in India. His main argument was that the Macedonians were exhausted and no longer had the will to continue conquering. Thus, Alexander would do better to freshen his troops. Coenus also reminded Alexander that his luck had held out, but that he should not try to stretch it too far. Alexander's reaction was similar to that of a spoiled child.