We, Trek Brasilis site, are betting on Garth of Izar. he's a strong character, underexplored, great strategist. It seems to me that he is human from another world, Izar. The writers can show him with more physical strength or others superhuman qualities because of the planet.
Wasn't he much older than Kirk though? Kirk talks about Garth being his idol in academy. That might explain Peter Weller's character (there's also some resemblance between Weller and Steve Ihrat who played him on the show).
I think the Garth theory makes the most sense if the Enterprise goes to Elba II to release Garth sort of like in that episode in order to help against Cumberbatch's character.