Jacob is a Roman demigod or legacy at Camp Jupiter.
The Heroes of Olympus
The Son of Neptune
Jacob is the eagle bearer, or Aquilifer of the Twelfth Legion. It's supposed to be a great honor, but given that it involves carrying an empty pole — the Eagle having been lost in Alaska — Jacob is less than thrilled and quite embarrassed by his assignment.
It is unknown whether he is still the Eagle bearer after Percy Jackson, Hazel Levesque, and Frank Zhang restored the eagle following their quest, as it is never mentioned.