Warning: This post contains SPOILERS for Eternals.
Harry Styles' Starfox is now part of the Marvel Cinematic Universe after Eternals' post-credits scene, which saw Harry Styles make his debut as Thanos' younger brother.
Starfox's introduction in Eternals came during the film's mid-credits scene. The scene saw Thena (Angelina Jolie), Makkari (Lauren Ridloff), and Druig (Barry Keoghan) flying through space on the Eternals' ship, the Domo. This takes place a few weeks after Eternals' ending as the trio continues to traverse the cosmos in search of other Eternals and tell them about their true purpose and origin. It is while they are wondering what happened to the other Eternals last seen on Earth that Thena and Makkari hear strange noises aboard the Domo. That is when Pip the Troll (Patton Oswalt) and Starfox teleport onto the ship and offer their help in finding the other Eternals.
Although Starfox's introduction in Eternals is meant to set up his bright MCU future, it also raises a big question about where he's been so far. This is an even greater question when the events of the Infinity Saga are considered. As the brother of Thanos, it would've made sense for Eros to get involved in the first three Phases of the MCU. Instead, he's completely absent from the Infinity Saga and all that came after it before Eternals. There isn't any explanation provided for where Harry Styles' Starfox has been before in his quick scene, although some details about his MCU past can be surmised. It would make sense if the beginning of Eros' MCU story was on Titan, but then they had to find a new home once Titan was destroyed after not listening to Thanos. This would be when Stafox could begin exploring the galaxy and going on adventures of his own, none of which crossed over with the Infinity Saga's events.
The lack of information on Harry Styles' Starfox MCU past makes sense, as does his absence from the Infinity Saga's events. The character wasn't part of Marvel Studios' plans at that point, so there hasn't been a need to explain why Eros didn't get involved. Furthermore, it is in line with Starfox's character that he wouldn't get involved. Thanos is a cosmic conqueror who relishes power, but Starfox is not concerned with such ideas. He's more focused on having a good time and finding new romantic relationships than the fate of the galaxy. So while his brother was out there attempting to collect the Infinity Stones, Starfox was enjoying his life and power. This would be when he came to meet Pip the Troll and eventually learn about his own connection to Eternals and what the Celestials are up to.
As the MCU continues to utilize Harry Styles' Starfox after Eternals, his future appearances can further explain what he's been up to. This could include some reveals that Starfox secretly played a role in the Infinity Saga, but the majority of his story should be focused on how he'll impact what comes next. The exception to this could be if Marvel Studios has any interest in an Eternals prequel starring Harry Styles' Starfox. Such a movie or Disney+ show could detail his whereabouts during the Infinity Saga while also informing his MCU future.
