Agents of SHIELD has revealed that Sarge actually is the real Phil Coulson... sort of. When it was announced that ABC and Marvel TV were bringing back the series for a sixth season, one of the biggest questions fans had was whether or not Clark Gregg would be involved. The season 5 finale showed a clear end for his longtime role as Coulson, but it was also difficult to imagine the series without him. It was eventually confirmed that Gregg would appear in season 6 but as a new character, Sarge.
Sarge looks exactly like Phil Coulson, but that is where the similarities stopped - or so Agents of SHIELD wanted us to believe at first. It was revealed that he's hundreds of years old and goes from planet to planet killing creatures known as Shrike, who are under the command of Sarge's archnemesis Izel. Sarge's arrival on Earth left the SHIELD team bewildered by his familiar face, and it was recently revealed how a centuries-old alien looks exactly like the former Director of SHIELD. When Coulson contained a time rift using the Time, Space, and Creation monoliths in season 5, their combined power created a copy of Coulson and sent him back in time and to a different point in space.
The body landed in Izel's dimension, where someone like her named Pachakutiq took possession of the body and formed this Sarge identity. Recent episodes confirmed that the memories that Sarge has are from Coulson, but that they were not entirely clear before. He now remembers Melinda May (Ming Na-Wen) and Daisy Johnson (Chloe Bennet) but as his wife and daughter. This was believed to be all that he could recall from Coulson's past life, but the most recent episode "From the Ashes" proves otherwise.
The penultimate episode of season 6 featured Daisy struggling to come to grasp with the possibility that Sarge could be Coulson in any way. She saw him as a weapon to use against Izel that would also rid the team from ever having to see the familiar face again. Daisy snapped Sarge's neck with the hope that more memories of Izel's plan and their previous life would surface. It initially seemed as though all this did was make Sarge more powerful, as he knocked down a wall with his bare hands. Then, Daisy's hope of more memories coming to Sarge was confirmed, just not how she expected.
Near the end of the episode, Daisy goes after Sarge with the plan to kill him using a specialized sword Sarge's team brought with them to kill Shrike. She fails to go through with it, though when Sarge calls her Skye. While Daisy is her real name, Skye is the name she used to go by before learning about her actual past. This moment confirms that Sarge has deeper memories of Coulson, as Agents of SHIELD hasn't referred to Daisy as Skye since season 2. This piece of knowledge and more like them are in Sarge; they're just buried.
Not only is Sarge starting to remember more about Coulson's past, but he's also beginning to act more like him too. While he's still more aggressive than Coulson ever was, Sarge's plan before Daisy confronting him was to sacrifice himself to take down Izel. As noted by Daisy, a sacrifice play is exactly the type of move that Coulson would've done. Its also a stark contrast to how Sarge let his own team members die for the greater plan just a few episodes ago, but is now the one ready to pay the ultimate price. May already had faith that the Coulson she knew and loved is still in this body and now Daisy believes it too.
But, Sarge having more of Coulson's memories and acting more like him does raise an important question that Agents of SHIELD will also need to answer. If Pachakutiq/Sarge took hold of Coulson's body, could it also leave the host behind, leaving just the copy of Coulson? We haven't seen any evidence of this, but since Sarge and Izel's race are inhabiting bodies in this realm, Coulson may be all that would be left if Pachakutiq/Sarge vacated the body. How or why that would happen remains to be seen, but theoretically, it could happen and leave the show with the real Coulson ahead of the final season. Now we'll just have to see what exactly Agents of SHIELD does during the last episode of season 6 to wrap this up or if this plot will continue into season 7.