I haven't read this yet, and I just started the anime. Can I ask who knows he's human?

this is a spoiler especially if you've only started the anime.
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the grandfather sullivan, opera, balam, bachiko, ameri (based on her own deduction), ameri's father, baal, kirio and ocho so far i think???

Kalego and his brother 2.

Ameri doesnt really know though? She thinks he's odd, but kinda gaslit herself into believing he's not human.

Thank you!! What about Asmodeus? He has appeared a lot in 2 episodes

no

ameri "knows" based on the idea that iruma can read the 'prohibited texts' = first love memories manga and how the sakura tree showed up in diabotany before they started doing their daily readings.

she even deduced that iruma couldn't possibly have relatives since he's a human to end up as "irumi" the akudol version of him thus why she definitely knows iruma is irumi as well.

it isn't explicitly revealed to her but again, she "knows".
she only tries to gaslight her father to keep iruma safe for the most part.

you're right, no wonder i felt like i was forgetting someone