In the comic, an entity goes through the rift between Endless Universe and Riftborn / Time Lords geometric Universe. Inside it is mixed with many geometric shapes. The final page sais "Thus, I became me".

Who is it?

First I though it might be one of the UC heroes. But I don't remember any Hero that would be tied to Riftborn,

Then I though that may be that's the whole Choir? It assimilated a whole bunch of shapes, so it could acquire many voices. Yet from the Riftborn Campaign and their prologue video it seems like the rift occured very recntly and is expanding very rapidly. On the other hand, the Choir is very-very old, older then Endless themselves.

Am I missing something?

The final line is actually "Thus, I became we." My reading of that line is that after their contact with the blight in the Riftborn universe, the Umbral Choir's once unified consciousness split into individual personalities, which is what they struggle with at the beginning of their quest.

However, that's just my theory. "Word of god" on this can only come from Slowhands or StoryEngine. (And they'll probably tell you it's deliberately vague.)

I thought Frogsquadron wrote the story.

I did not write any of the comics. I helped with the story in the case of most comics, but the name on the front cover is always the one of the writer foremost involved in the writing. In the UC comic's case, it was StoryEngine. :)

I mean