She probably knew Iruma's parents and gave them the book they used to summon Sullivan and sell their child.
What if she plotted for them to send Iruma to Underworld to meet Ameri, because she liked Iruma.
Nah, i'm just messing with y'all 
After all, she probably got memory erasure treatment, i would imagine.
I see the Iruma's mother theory is still popular in the comments. I wonder whether the author regrets a bit making the design that Iruma like (assuming the theory is wrong which I think).^^
I'm pretty satisfied with another theory - that Nishi drew Merize Iruma-shaped just to (pardon the tautology) draw a parallel between Ameri/Iruma and Henri/Merize dynamics. So, it's probably just an artistic device, and they shouldn't be considered that similar looking lore-wise.
But it wont be my L if siblings theory will turn out true
There is 0 chance she is Iruma's mother.
As much as they act the same, it's either coincidence.. or reincarnation, which honestly is just as bad as them being siblings. It could also just be, only humans with their type of personalities are allowed in the demon world to begin with. While not overtly established, it was pretty much given based on how Iruma talked about them, they were his real parents. He was just a baby, then forced to work the moment he could walk. I doubt people that selffish would care for a baby for a year or 2 before the child "became useful".
Just for the arguments sake...
Iruma's impression about his parents doesn't really prove that they are blood related. If they aren't, those scums would've really liked for him to think that they are his real parents.
And do you mean that they only cared for baby Iruma, cause they were his real parents (despite being the scum they are)? Sure, but they could've adopted him at whatever point in time before he "became useful", and as to why - maybe some kind of money scheme or whatever (i mean, it isn't a solid theory by any stretch, but neither is your theory about "personality"

).
How does everyone even think that Merize is Iruma mother ??? Iruma have blue hair and we kinda see his parents before and we see that he worked in her office before (assuming she is the author of first love memory)
https://forums.mangadex.org/threads/mairimashita-iruma-kun-ch-440-merize-4.2613096/post-29652609
was slashed on his shoulder or something by one of them
I think it was Azazel himself that slashed him. As to everything else, well, we'll have to see it for ourselves in the future chapters.