Author's Note:
If I were to ever continue this, I would re-write it from scratch.
The development of this fanfic did not quite work out as I wished to.
I'm a bit ashamed of what's published and wish I could delete/hide it for a future re-writing, but I keep it around in this form since it's still decent and some people might want to read it, even if just for having Mordred. Keeping the comments for reference would also be useful to me.
As it is, Mordred might seem like an Emilia simp, like some readeers pointed out. I don't thinks it's too bad, but I don't like the way it is also.
As I came up with the idea of Mordred and Emilia actually being the pairing here, which wasn't the thing at the start, I thought of Mordred being more violent and rash as Mordred's spirit affects him, whereas both Mordred and Emilia would help each other grow (like when Mordred bursts into the loothouse and threatens everyone on first sight, but Emilia would want to negotiate first, calming him down), and it looked well in my mind at the time.
But I removed these flaws from Mordred, making him more reasonable then the Heroic Spirit, so it turned into a pure one-sided help of Mordred to a foolish and useless half-elf, just like in the original.
And Mordred choosing Emilia also came out quite suddenly with the current development, even though the reasons were good (her actual freaking OP powers and Mana, her potential and possibility of being related to Satella, even though she's naive and useless now).
So, I'm not entirely proud of how the story turned out, but I'm leaving it here.
To re-write, I would probably need to catch-up on hundreds of novel chapters of the original, but I don't feel like it right now.
Honestly, I don't even like the Re:Zero verse that much. The main reason I chose it for a the background here was because the fantasy world and the magic system in Re Zero was so simple and basic that it made inserting a Fate character like Mordred very easy and fitting, and Mordred's abilities could be easily translated there.