Agent clark (new guy) looking for chief to gather data only he knows and for his help to help save arbitars home planet.
First of all it is agent Locke
Second of all, we need to first go to the origin of the e3 2013 trailer. How did Chief get here? We all know at the end of halo 4, Chief returned to earth, had his armor stripped and Cortana left Chief for good. What if he snuck out of the infinity to look for any ways to find Cortana again, find a source which brings him closer to regain Cortana or went to find Halsey to rebuild Cortana. He wears a robe(probably so he doesn't get identified by any Storm Covenant, UNSC looking for him or Prometheans). He also has a visible crack on his visor which may or may not be explained in the halo escalation series(you should check that out). The Promethean bird suddenly stands in Chief's way. As the hood of his robe gets blown of by the Promethean's activation protocol, Chief clutches onto Cortana's chip(which has been made into a bracelet). This probably means he has found the source or a clue of Cortana's return. This is where Agent Locke comes in. If you Look at the first announced cover art for Gaurdians there is a few things that we should know about Locke. First of all he has an O.N.I symbol on his armor. This tells you he is sent to search for Chief or kill him. Also Locke is the main character of Halo Nightfall, which will give you more clues about the story of Guardians. Finally he is sent by the Arbiter, who once formed an alliance with Chief. In the Halo MMC trailer he told Locke that the universe depends on him and to know him he needs to look back at his previous accomplishments. So there you have it. Feedback to your reply and more possible theories.