>>283470442
>but I don't see how the "witch" Bern has anything to do with the witches in Umi.

I assume you don't mean the anime which basically drops the entire plotline and left just a random namedrop?

Still, even in the VN, the actual part that ties into Bern's existence is rather random. It's basically down to this one line Rika has near the ending of Saikoroshi, about her other self splitting from her once she decided to give up looping and the Bern name, and this other self still existing and is playing with fragments in the upper realms. Rika just randomly says this while lying on bed, while the story itself doesn't show anything happening.

The secret ending of Matsuribayashi also showed her being able to change the events of a world.