Honestly, that's what adaptations should do: expand more on some bits of the story that couldn't be shown in another medium.the manga seem to try to expand on how this 2 come to be.
cause in novel(web novel at least) it just off hand mention them being close in 1 chapter
The wn is basically a booklet of information that connects more than a "novel" itself, so much so that it only has thirteen chapters (although the side stories have more extended content) and most of the wn is narrated by someone, or by Remilia (and honestly, when reading it, it becomes very clear that she borders on sociopathy. That is, the way she treats people, situations, as well as her revenge is very methodical and brutal)~ ✨the manga seem to try to expand on how this 2 come to be.
cause in novel(web novel at least) it just off hand mention them being close in 1 chapter