Who is Phastos? - The Eternals

Phastos is the Tony Stark of the Eternals, meaning he's the brains behind all their weapons and inventions and is known to be the smartest of the Eternals. He not only helped the Eternals in technology but as he worked with humanity on nudging them throughout time advancing their tech. In the comics he was the one that was always looking for happiness in life, wondering if there was something more to it. 

In modern times when his mind was wiped and he believed he was a human named Phillip Stoss, he worked as an automotive engineer in Germany and was still on the search to find that special something in life. When Ikaris tried to help him realize what was going on so he could help them take down Drui who was doing some naughty stuff, he didn't want to help, but Ikaris didn't want that to happen so he forcefully "woke" him up. 

He had a moment with Thanos later, where he blackmailed him into resurrecting him so he could destroy machines that resurrected Eternals. This pretty much gave Thanos access to kill Thena's father and other Eternals, and he screwed up bad (obviously). So Phastos had to repair the machines but failed epically. He reached out to Ikaris who came and helped and was able to fix them but while Phastos was minding his own business as Ikaris fixed his mistake, Druig entered his mind and erased the method Phastos had on killing Thanos. 

This is about the extent of knowledge we have on Phastos however, in the upcoming Eternals movie, he will be revealed as the first gay superhero in the MCU!