The Investigator is a construct of the protomolecule, where it uses an edited version of Detective Miller's persona, to look into the disappearance of the Protomolecule's parent race. The Investigator interacts with James Holden through the protomolecule's manipulation of Holden's brain; in which he essentially hallucinates the Investigator.
At some point, the protomolecule decides to allocate resources toward the replication of Miller, who was a talented detective and could further the protomolecule search for the Ring Gate Builders
The Investigator first appears to Holden at the end of Caliban's War after The Ring Gate has made it to its location past Neptune's Orbit. The Investigator, taking the form of Miller, mysteriously warns Holden about "Doors and Corners". Three days later, Holden finds out about The Ring Gate getting breached by a rogue belter who uses Gravity assists off planets to gain the required speed to slingshot out to the Ring Gate.
Initially, the Investigator shows himself to Holden in a confusing state where he makes little sense, using non-sequitur phrases which do nothing but scare Holden into believing he is getting manipulated by the protomolecule. This is further exacerbated by the blind sound engineer giving a 3D model rendering of Clarissa Mao who is mistaken for her sister, Julie Mao.
The first coherent conversation between Holden and the Investigator happens when Holden makes his approach alone to the Ring Station. Still confused as to where the Investigator comes from and why he is there, Holden asks why he is being framed by Julie Mao. the Investigator replies "She's dead kid, I don't want to talk about it". The Investigator also explains that he was unaware of appearing to Holden before in a more confused state, stating that it "scares the shit of him". The Investigator later explains that when he becomes powerful within the protomolecule construct, it will kill him, and he is reborn within the construct. He says he's "good at dying".
After arriving on the Ring Station, the Investigator opens up the pathways in the Ring Station. Once Holden arrives in the room with the console, that only physical entities like Holden can approach and not the Investigator. After seeing the recap of the end of the Civilization that was the Ring Gate Builders, The Investigator explains he can shut down the Ring Station's security protocols if Holden can get the fleet of ships to lower its power out put enough.
Once all the fleet ships and finally the Behemoth shut down their reactors, the Investigator shows Holden one of the planets on the other side of the gates, while in the hallucination which seems quite real to Holden, The Investigator explains there is a habitable planet on the side of every gate.
It is soon revealed that the the Investigator permanently shut down the dampening field affecting the speed of the ships in the Slow Zone, while also opening up the unopened gates. This is effectively the second time Miller saved the Sol system.
The Investigator explains to Holden that he wants to continue to explore and see what really happened to the Ring Gate Builders.
Due to events not related to the protomolecule, the crew of the Rocinante make their way to Ilus to help mediate an issue with the Royal Charter Energy, or "R.C.E.", colonists and the Belter Squatters. When the Rocinante's crew arrive, with Holden and his "Miller Ghost," who by proximity to the Planet unknowingly to Holden causes the planets Ring Gate Builder assets to begin to wake up, like a key turning on a car. In space, this causes the ship reactors to shut down, forcing them to rely on battery power. On the planet, they first see nearly broken Protomolecule Scarab Robots that have be activated but are succumbing to their age. Then, the reactors deep under the sea, inactive for billions of years, blow up causing a massive planet-wide storm which forces the people at First Landing to retreat hastily to the nearby towers built billions of years earlier.
While this all happens, the Investigator insists that Holden go to a crater that is hundreds of miles away to investigate what is in that crater because he can't see within its depths unlike the rest of the planet.
After Elvi found a remedy for the Green Blindness, Holden, who has been defending the camp from death slugs, passes out in a sleeping bag after a multiple-day toil. In his sleep, The Investigator insists that Holden go deeper into the tower and enter its planet-wide transit network. After a little cajoling, Holden makes his way to the Transit Network, where he passes out.
After a long trip to the mysterious crater Murtry side-tracked Holden's attempt to enter that crater and his attempt shut down the weapon inside. Murtry has made his way up to the crater in a land vehicle. Eventually, Holden finds Elvi to whom he delegates the task of shutting down the weapon with the Investigator, now operating "Miller Bot".
After some instruction on where to go from Elvi, "Miller Bot" starts the shutdown process of the protomolecule Network on Ilus. He explains that anything that is part of the protomolecule's race will instantly die if it enters the circle in the middle of the room. Soon local defenses come to try and stop "Miller Bot" and Elvi. While Elvi successfully avoids dying, the "Miller Bot" is severely damaged and The Investigator is forced to unhinge itself from "Miller Bot". By eventually doing so, she reveals that inside the "Miller Bot" is a heavy oblong-shaped egg creature. the Investigator asks Elvi to pick up the heavy egg and place it within circle essentially destroying it and all the protomolecule assets he can find thus it destroys this instance of him.
Holden, after returning to the orbit of Ilus, decides to look through the area where the protomolecule Hybrid was digging into the ship, and finds a polyp the size of a fingertip. This part of the protomolecule had been how the protomolecule always seemed to follow Holden.
The Investigator's primary mission was to search for Ring Gate Builders. The Investigator was largely indifferent toward the survival of the human race, other than Holden who is needed as a human conduit for the search. The construct of the Investigator only helps Holden while it serves the protomolecule's programming. The Investigator explains that he chose Holden because Miller liked him.
- ↑ While not said directly, his sacrifice could have been made to end the suffering of the hundreds of thousands of people who died on Eros but lived on because of the protomolecule.
- ↑ The preceding assertion disregards that the creation of the Investigator was a consequence of the protomolecule achieving the consciousness that would make the decision to sacrifice itself; however, that consciousness would not be possible until after the gate was completed and the necessity of the Investigator was determined by the protomolecule. Furthermore, the sacrifice was only possible after the Investigator had a human counterpart carry the egg to the hole that was on Ilus/NewTerra