I don't think the knight is a love rival, from what I see, I think reincarnation is really a thing in the country though maybe only as a legend with no proof
Maybe saying that I want to see you again the next time I'm born is a big thing in a country that probably believed in reincarnation which why the knight was really surprised and called her a foreigner for not knowing the meaning of her words
In the flashback, the knight was reminded of a woman with brown long hair. If our MC is the reincarnation of the lover in the legend, what if she's also the woman we saw in his flashback.
Another thing, it's been repeatedly said how MC wanted and see the knight as an older brother. So what if, in their previous life they really are siblings
This is just me wanting to blabber my mouth cause I love this series
Thank you for the chapter
Edit: nvm reincarnation may not have anything to do with this. He actually had a little sister