Sean Miller, also known as Astro, is a U.S. Recon Marine.
Through unknown circumstances, Sean Miller joined the United States Marine Corps, where he gained the call-sign Astro.
Astro was among Shane Schofield's team, and was one of the survivors at the end of the book.
Astro accompanied Paul Robertson of the CIA to the Burj al Arab Tower, as part of the United States' genuine wish to participate in a meeting concerning the approach of the Dark Star and the rebuilding of the Machine. Robertson assigned Astro to Jack West Jr.'s team, and as the tower was struck by a plane, Astro helped Jack and Stretch gather up their notes before parachuting off the building's helicopter pad to safety. As their expert on the Machine, Max "Wizard" Epper, was being held at Xintan Prison by the Chinese, Astro helped supply the team information about Xintan so that they could break him out. Astro noted the risk they were taking to help Wizard, and asked Jack he would do the same if he were captured, to which Jack said he didn't know him well enough yet, but would see based on his loyalty.
After Wizard and Tank were liberated, Astro helped commandeer the Xintan Prison's chase chopper, and accompanied everyone to Laozi's Tomb to retrieve the Philosopher's Stone. When this was done, Astro joined the second meeting on Mortimer Island, where they were given more information about the Machine. As the team were making their way to the first Vertex near Abu Simbel, Astro was questioned about his country's knowledge about the Japanese's reputed pocession of a Pillar by Iolanthe Compton-Jones, who believed that Roobertson had informed him of America's attempt to seize the Pillar after World War II, but Astro was clueless.
At Abu Simbel, Astro helped the others search for the Vertex entrance, and through his underwater motion sensor belived to only spot crocodiles. Once Jack lay the Pillar, the team were surrounded by British forces, and Astro cursed himself for not realising that they were what had set off his sensor. Iolanthe left them to die by having him, Pooh Bear and Alby Calvin handcuffed to their sinking zodiac, but luckily Jack and Zoe Kissane arrived and freed them. In the chase to catch up to the Halicarnassus, Astro gave Jack his Armalite MH-12A Maghook to use. However there were forced to abandon their pursuit in order to allow the others to escape, and after ditching their vehicle they were apprehended by American forces, who drugged Astro.
Astro was taken to a mine in Ethiopia, still under the drugs' effects, and Wolf had two guards hold Astro up as he showed him to Jack, making it seem as though he had been working for Wolf all along. Once Jack was apparently killed, Rapier asked what should do with Astro, and Wolf replied that when Astro woke up, he would be given the choice to join them or die.
Astro woke up at the base on Diego Garcia, and was told that Robertson - who was actually in league with Wolf, and not the genuine American government who had authorised their association with Jack - was not supposed to have let him join Jack's team. Once he recovered he was drafted into the CIEF forces looking to place the rest of the Pillars.
When Wolf launched his assault on the Third Vertex's entrance at Hokkaido, Astro was among his forces. Astro tried to help Rapier, carrying the Philosopher's Stone, get to the entrance, but was unable due to the heavy fire from the Japanese. Luckily Jack, Zoe and Lily arrived and got Rapier to safety, and Astro was surprised to see Jack. Despite this, he followed him into the Vertex along with the other CIEF men, following his orders to open fire on the Japanese inside the Vertex without question. During their journey through the Vertex, they came across the trap containing the Vertex's Pillar, and were forced to wait until they could get access.
During their time waiting, Jack spoke to Astro, who said that he didn't remember being in any mine and had been told that Jack had been planning to kill him and take the Pillars for himself. Jack realised that Astro had been tricked into infiltrating his team by Wolf and Robertson, and tried to tell him this. Astro was reluctant to believe it, but told Jack that he and everyone else was ordered to kill him once the Pillar was laid. He was part of the team intended to retrieve the Pillar from the trap, but was unable due to the Japanese snipers, and Jack got the Pillar instead.
Upon making it to the Vertex, Astro helped secure the boats Wolf and Jack were in using a cable winch with Rapier while they tried to lay the Pillar. However a final attack from the Japanese saw Astro being heavily wounded. After the PIllar was set, Rapier assessed Astro's condition, and Wolf decided to leave him behind. Thankfully for him, Jack was not so callous, and remained behind to help him back out of the Vertex, and Astro thanked him for his help despite their being on opposite sides, and realised that Jack had been telling the truth. When they got out of the Vertex, however, they were captured by Russian forces.
Astro, along with the others, was taken to an old observatory belonging to Carnivore, who had decided to step in on the mission to rebuild the Machine. Astro was placed inside one of his tank in the Living Tombs despite his wounds to be used as leverage for Jack's help, along with Zoe, Pooh's father, and Alby and his mother. When Carnivore abandoned the base, Wolf escaped his tank, and Astro was hopeful that he would release him, but Wolf didn't.
While Jack defeated Carnivore at the last Vertex and repelled the Dark Star, Astro and the others were freed from their tanks by the rest of Jack's team.
- He was given the call sign Astro by his fellow marines due to his love of science fiction novels.
- Astro was the first character to appear in both the Scarecrow and Huntsman series, which at the time suggested that they were both set in the same fictional 'universe'.
- With the events of The Four Legendary Kingdoms, it is now confirmed that the characters from both series exist in the same universe.