Thomas Jane's Miller has been one of the most important characters in The Expanse, and while the character's story appeared to end with season 4, the detective could yet make a return in season 5 and beyond. When The Expanse begins, Josephus Miller is a secondary protagonist whose journey dovetails with that of James Holden and the Rocinante crew. A noir-ish, hardened detective with a heart, Miller isn't above a bit of rule-bending if it gets the job done, but the man has principles, and refuses to be drawn into the political corruption of his Star Helix employers. Miller becomes obsessed with the missing persons case of Julie Mao, and continues to investigate her disappearance even after being fired.
Miller's investigation leads him to Eros, where he ultimately realizes that Julie is still part of the Protomolecule-infested asteroid. Having fallen in love with Julie from afar, Miller forfeits his own life to find her. Exposing himself to infection, Miller discovers the Protomolecule's manifestation of Julie piloting Eros towards Earth, but after learning about her through his detective work, is able to comfort the scared young woman and divert Eros into crash-landing on Venus instead. Miller is hailed a hero for his actions, but his physical form is killed in the incident.
Just like Julie, however, the Protomolecule has other plans for Miller. Although the Protomolecule wasn't designed for organic material, the blue substance is programmed to dissect, learn and assimilate whatever will prove useful. After Miller dies on Eros, his consciousness lives on within the Protomolecule, and Thomas Jane returns in The Expanse season 3 as The Investigator. Although The Investigator also appeared to "die" in The Expanse season 4, that doesn't completely rule out a return for Miller, especially with Jane confirmed to be directing an episode of the forthcoming run.
How Miller Became The Investigator
Within the obscure inner workings of the Protomolecule, Miller's return as The Investigator is one of The Expanse's biggest curiosities. Fortunately, The Investigator does at least try to explain his existence to Holden. Like Julie Mao before him, Miller died surrounded by Protomolecule, and thus his consciousness was assimilated into the wider network of this intelligent alien goop. Meanwhile, a Protomolecule Hybrid soldier attacked the Rocinante, leaving behind a small, undetected node on the ship, and this allows the Protomolecule to mentally reach out to Holden. Much to his confusion, Holden begins hallucinating visions of Miller that converse with him and guide the Roci crew into opening the Sol system's Ring Gate.
Because the Protomolecule and Miller essentially became one upon the former cop's death, Holden's visions have the physical appearance, memories and personality traits of Miller, but this character identifies itself as The Investigator - a mental avatar for the Protomolecule. Parallel to how Miller was doggedly investigating the disappearance of Julie Mao, the Protomolecule is trying to uncover the mystery of what killed its creators, but Holden's help is needed to achieve this, and the Investigator appears to Holden in the guise of Miller to push the agenda of the Protomolecule. Sometimes this aligns with Holden's own interests, and sometimes it very much doesn't.
The exact science of The Investigator's link with Holden is left deliberately vague in The Expanse, but the vision is described like a burst transmission "reaching out" 113 times per second directly from the Protomolecule to Holden's brain. Each time The Investigator appears, an entirely new construct is generated, and while each incarnation looks like Miller, it's implied that The Protomolecule could've taken another physical form in Holden's mind, opting for Miller due to the bond between both characters.
What Happened To Miller In The Expanse Season 4
In The Expanse season 3, The Investigator helped Holden open the Ring network, and save all of the ships trapped within the Protomolecule's slow zone, thereby earning a measure of Holden's trust. In The Expanse season 4, however, Holden begins to question where The Investigator's allegiances lie. After landing on Ilus, the Rocinante crew discover inert Protomolecule structures scattered across the planet and reaching deep below the surface. Guided by an increasingly erratic Investigator, Holden begins inadvertently activating these systems, putting the planet's colonists and his own crew in grave danger.
Just as the situation on Ilus looks darkest, Holden receives a vision of Miller that's the genuine article, not a Protomolecule copy. Miller reveals that his consciousness has been present all along, being used and recycled by the Protomolecule against his will. Inspired by hitchhiking inside Holden's head, Miller was finally able to "outsmart the jailer" and appear to Holden as himself, rather than The Investigator. While in service to the Protomolecule, Miller discovered a place on Ilus that his hat-wearing counterpart was vulnerable to, and he encourages Holden to go there for both their sakes. Together, the pair find a black hole-esque bomb created by the same mysterious species responsible for wiping out the Protomolecule's creators. Essentially an anti-Protomolecule construct, the weapon is capable of knocking out the reactivated systems on Ilus, as well as finishing off what remains of Miller's Protomolecule consciousness.
Barely fighting off the Protomolecule, Miller inserts himself into a physical body (several lumps of nearby metal), connects to the wider Protomolecule presence on the planet and makes contact with the bomb. This seemingly destroys everything that Miller was hooked up to, including the assimilated souls from Eros. And just to make doubly sure he can finally rest, Miller advises Holden to clean up the remaining Protomolecule attached to the Rocinante, which he duly does. The remaining Protomolecule is sent to burn and Holden bids goodbye to a dear friend.
How Miller Could Return In The Expanse Season 5
It's suggested that Miller has finally (properly) died and Holden's visions of The Investigator will cease, and in The Expanse's original book series, neither The Investigator nor Miller have returned since the Ilus fallout. However, The Expanse TV series has been known to meddle with its source material, and does perhaps tease a future return for the detective. Speaking with Naomi, Holden rues his final words to Miller, to which Naomi asks what he would say if given another chance. The question goes unanswered, leaving the possibility of Holden rewriting his parting speech to Miller hanging in the air, potentially setting up a future encounter between the two.
Crucially, the Protomolecule's presence in The Expanse is far from over. Although the days of rogue blue stuff taking over entire asteroids may be over, the Sol Ring Gate is still up and running. The gate itself was constructed from the Protomolecule on Eros which crashed on Venus, gradually took form, and then flew to the other side of the system before transforming into a gate. Miller hoped that by using the abandoned bomb on Ilus, he could bring peace to both himself and the restless voices of Eros still connected to the Protomolecule. Thematically, this makes sense, but the Sol system's Ring Gate runs off the very same Protomolecule that once infested Eros, and as long as the bomb's effect didn't resonate that far, the potential for Miller's return remains. The Protomolecule is a widely interconnected network of information, and it's impossible to know whether Miller's self-sacrifice destroyed all of his lingering consciousness, or whether other bits of remaining Protomolecule from Eros could reconstruct Miller in the future. The decision is ultimately up to The Expanse's writers. Ilus provided a fitting end for Miller, but there are built-in methods to bring the character back easily enough if necessary.
It may be more emotionally resonant for Miller to remain dead in The Expanse season 5, but that doesn't necessarily prevent Thomas Jane returning as The Investigator. As long as the Protomolecule is still "out there" in some capacity, it will continue to seek the next clue in the case, probing into the demise of its creators. In The Expanse season 5, the Protomolecule could "reach out" to a different character (perhaps Elvi Okoye, who has already established a connection to it) but still take on the physical appearance of Miller.