Starfox being Thanos' adopted brother also more easily explains the differences in their appearances. While Thanos' purple skin is never really explained in the MCU, it's the result of having Deviant genes in the comics. (Originally, all Titans had purple skin, including Starfox, but that has since been retconned.)
The confusing part is that Thanos is affected by the Deviant Syndrome which gives him his signature purple look, despite not explicitly being a Deviant. And so, Thanos is very much an Eternal like his brother Eros.
An Eternal getting a Deviant-like mutation was later dubbed “Deviant Syndrome,” or the way Thanos looks with his purple skin and ridged chin. (Of the two sons, he's the only one with this condition; Eros looks like a regular Eternal, something that Thanos naturally hated his brother for.)
This certainly sounds like his Deviant Syndrome from the comics. With this in mind, it makes sense that the MCU's Thanos could be an Eternal with the Deviant gene. This would explain why he doesn't look like the characters from Marvel's upcoming Eternals film or his brother.
That comic revealed that, fearing his own potential, Starfox has always limited his powers, but he is actually "the equal and opposite of Thanos" on a genetic level.
He was born on Titan where he is the son of Mentor and the nephew of Zuras. Where Eros serves as the superhero Starfox, he is opposed by his mad brother Thanos, much like the rest of the universe. He has the power to psychically control other people's emotions.
Starfox being Thanos' adopted brother also more easily explains the differences in their appearances. While Thanos' purple skin is never really explained in the MCU, it's the result of having Deviant genes in the comics. (Originally, all Titans had purple skin, including Starfox, but that has since been retconned.)
Thanos' biggest enemy may not be the Avengers, but the Merc With A Mouth - Deadpool himself. The Mad Titan's cosmic opposite is revealed as he battles Deadpool over a star-crossed romance. Even with Thanos' admiration for Deadpool's resilience, their rivalry is likely confined to the comic world.
Thanos' purple skin was completely unremarkable upon his debut as it was strictly uniform to his Titan species, though his appearance earned a much greater meaning when he was retconned into being an Eternal which simultaneously caused his family members to lose their purplish hue.
Arishem may have calculated a simple cost benefit analysis and determined that opposing Thanos given all of the aforementioned conditions simply wasn't worth taking the risk to himself. There is also the matter of the battleground itself.
All Titans were initially shown being purple-skinned like Thanos in Iron Man #55. The idea of Thanos being different-looking from the rest of his people wasn't established until six months later in Captain Marvel #27.
At the peak of their battle, King Thanos calls Thanos by his intended birth name: Dione. The story of how Thanos earned his name begins on the planet Titan. Thanos was born to A'Lars and Sui-San, members of a race of aliens called Eternals.
So despite having plenty of raw power, it was never enough for him to overpower his brother on his own. If Starfox's powers are at all similar to his comic counterpart, it's likely that he would have fared better against Thanos than most of the heroes in Avengers: Infinity War and Avengers: Endgame.
When Gamora finally does confront Thanos, she believes for a moment that she's killed him and breaks down, crying. An orange light passes over her--but it's fleeting. Whatever love Gamora has for Thanos is all but smothered by darkness.
As per the user, the director of Avengers confirmed that Thanos was most scared of Iron Man. “Not because of his strength, not because of his power, not because of his power, but because of his suit,” he added.
When Thanos got control over all six Infinity Stones, he became one of the most powerful beings in the Marvel Universe, though on his quest to obtain them, Thanos revealed that he was afraid to face the Hulk.
Created by writer-artist Jim Starlin, the character first appeared in The Invincible Iron Man #55 (cover date February 1973). An Eternal–Deviant warlord from the moon Titan, Thanos is regarded as one of the most powerful beings in the Marvel Universe.
Nebula appears in The Super Hero Squad Show, voiced by Jane Lynch. This version is Thanos' older sister. Nebula, based on the Marvel Cinematic Universe (MCU) incarnation, appears in Guardians of the Galaxy (2015), voiced by Cree Summer.
When she learns of Thanos' ultimate plan of destruction, she escapes from his control and joins the Guardians. In the comic books, Gamora is depicted as a green-skinned alien from the planet Zen-Whoberis.
Hercules is considered the physically strongest character in the whole marvel universe. Literally the strongest, even over characters such as Thor, Hulk, and the Destroyer.
However, Red Skull saw his true intentions, informing Gamora that Thanos was, in fact, weeping for Gamora, as he truly loved her as a daughter, and was heartbroken that he had to sacrifice her to get the Soul Stone. Disbelieving, Gamora could not comprehend that Thanos ever showed genuine love for her.
He is smarter than even the brightest of Earth's geniuses, including Reed Richards and Dr. Doom. Likewise, he can control time and the souls of the dead. Considered to be an equal to Zeus, Galactus, and Odin, Kronos remains the most powerful and strongest of Marvel's Eternals.
