The 31st-century variant of He Who Remains/Kang was named Nathaniel Richards. According to the Marvel Database, Nathaniel was a direct descendant of Reed.

Like many other of his fellow Marvel comic book characters, Kang has had different origins over the years, so for the sake of simplicity we're going to refer to Kang's discovery in What If...? Vol. 39.

Per Comic Book Resources, Nathaniel takes a DNA test in that comic book issue that confirms he's related to Reed, although the exact familial connection is never confirmed.

Now, here's where things get a bit complicated (thanks to time travel)! Reed's own father was also named Nathaniel. Why is this important? Nathaniel was a time-traveler himself, and he was also known as Kang in some universes, per Games Radar.