Did you know there's actually two Hagens from Norse/German mythology? One is the Nibelung brother from Burgundy, and he's much more famous, but the other Hagen was just as famous and popular in the High Middle Ages. He was Hagen of the "Holmrygum", a people located somewhere in continental Germany. His wife, a shieldmaiden, gave him the cursed sword Dainsleif, that would never be sheathed until he used it to slay his intended enemy. A certain King Heten (Hetel in Kudrun), with the aid of his bard Horant, elopes with Hagen's daughter Hilda. And according to various versions, the lovers either elude the angry father for several years, or they're immediately caught and reconcile for the time being, until some of Hild's valkyrie buddies decide to play a prank and urge Heten to abduct Hilda again, because a wife won at sword point is more glorious than a wife given away by her father. Either way, when Hagen catches up with the lovers, he draws Dainsleif in a rage, and although Hilda does calm him and Heten down, they're forced to fight because of the curse of Dainsleif. After a day of fighting, both Hagen and Heten and all their armies lie dead, so Hild blankets their island in a spell that essentially causes a time loop, where the men will revive in the morning, go through some incident that leads to them fighting again, and all end up killed by evening, only to go through it all again as Hilda renews the spell. It's a legend that's not much discussed in the modern day, but in medieval literature it has a bunch of versions and a bunch of mentions in other bits of poetry and chivalrous literature, and like the Nibelung story, authors created a bunch of fan insert characters to flesh out the armies, like Heten's follower Deor or Hilda and Heten's daughter Kudrun.
I did actually know the Hagen in Kudrun wasn't supposed to be "my" Hagen! The reason I included him in that one shitpost was because I had read somewhere (sorry for how vague this is but it was a good while ago -- I thiiiiink the English Wikipedia page for Kudrun might mention it?) that the structure and characters of Kudrun were meant to directly parallel and contrast those of the Nibelungenlied in some way, down to the descriptions used for a couple of characters. So, that was enough of a link for me to talk about it and make a joke out of the whole "... don't tell me, I won't get it until I read a translation of the text alongside an in-depth commentary of it, right?" of the situation.
However, I didn't know the other Hagen was so popular in continental sources, or about Kudrun being a later addition to his story. So, thanks for bringing that up! I hadn't really connected Kudrun with the story of Hogni, Hedin, and Hild, either, but you're ofc right that it does keep popping up in all sorts of sources. It sure is fascinating. But then, looking at the ways different continental, Scandinavian, and Icelandic traditions rework and add their own material to the same stories always is!