In Marvel comics lore, Starfox is another member of the Eternals – a genetically altered branch of humanity that left earth and settled on the Saturnian moon, Titan. Despite being raised alongside his brother, Thanos, Starfox developed a very different personality from the
genocidal Mad-Titan. Instead of yearning for the Infinity Stones and pursuing a mission to exterminate half of all life, Starfox became a carefree adventurer – much more interested in seducing various characters across the Marvel universe and adventuring across the galaxy. In fact, this independent, daring streak is one of the character's hallmarks, explaining why – despite his regular liaisons with key members of The Avengers – he rarely remains in one place for long.
The reason
why Thanos and Starfox look so different lies with their genealogy. Unlike Starfox, Thanos is afflicted by the Deviant Syndrome, giving him certain Deviant characteristics. This explains both his purple coloration, the deep lines on his chin, and his sizeable physique. Starfox, by contrast, retains the rest of the Eternals' more stereotypically human characteristics.
However, despite his connection with the Eternals' mortal enemies, Marvel has subsequently confirmed that Thanos himself is indeed a true Eternal. In
Eternals #6 it is revealed that, though he is different from both the Eternals and other inhabitants of Titan, he still shares a strong connection with other members of the Eternals team, including his brother Starfox. Therefore, despite the clear differences between Josh Brolin's brooding, muscular onscreen portrayal of the Mad Titan and Harry Styles' more mischievous, youthful appearance, there are understandable reasons behind the contrasts.
As for that MCU will do with it, I'm not sure. I suspect they'll keep the deviant strain aspect as to why he was physically alterred , but basically make him like an eternal.
