It's an AU, the main story is safe and sound and so are its characters
It's an alternate universe. It's not the same Reinhart and valeta.
It's an AU.
OG Valetta met AU Reinhart and he became obsessed. So he searched other AUs for a living Valetta he could keep as a pet since his was already dead.
If I had to guess, the author got comments on the LN/WN asking about an AU with this particular Reinhart, so they obliged and now it's depicted in this webtoon.
So it's not the one in the main story?
Nope
What kind of side stories is this ? Ruining the main story is kinda disappointing.