I said I'd give some feedback, and I'm rather late, and I'm sorry.
So the thing I found weird is within the span of just a paragraph, the PoV seemed to transition from the Merchant to the Nymph. I actually had to pause my mental movie and ask "the heck is going on?"
"newborn nymph wasn’t scared. For her mother" -> I would have combined this with a comma. I personally am not much of a stickler for grammar but others are. But this was something I noticed.
I like the achievement bit, and I like how the words just appear. I also really like how you specified the mana regeneration rate as "mana per day". I find it interesting that you never specified a maximum mana capacity for the nymph, and I predict that there is no cap, having just read Chapter 1.
"“I have ten mana, yay!” Jumped in joy Theanore." --> “I have ten mana, yay!” Theanore jumped in joy
That'll help.
Something that irked me a bit was how regen was used to mean "regeneration rate". Like I know some games do this, and so feel free to ignore my nitpicking, but I would have said regen rate. This is offset by you exploring "one shot mana rewards" vs "rewards to your daily mana allowance".
I find the idea of a Friend casting down Achievements and bestowing supernatural gifts to me if I complete tasks a little.... weird. Like I personally probably wouldn't call that sort of entity a Friend. But I really like how the SYSTEM is integrated in as an actual thing the nymph(s) (I'm assuming there're more of them) can talk to. Theanore seems very friendly outgoing and trusting, so if that's what you're intending it worked.
Overall, I'll be placing this on my reading list. I feel you could use a beta reader at the very least, but your update rate seems more insane than mine, so I understand if you don't. More power to you, by the way, in maintaining that.