For those who're confused

So by far this is how I understood the story:

We have Ikuro, Ranzo, Sachiko, and stupid Norihiko.

Ikuro our protagonist grew up neglected by his father and abused by his mother, his father was always spending his time in the "dungeon" where someone "like a mantis" or monster was kept being hidden; and his mother got sick in the head bc of that.

Fast forward in the present, mother died followed by the father. Now Ikuro discovers that "the monster" locked up in the hidden room behind their house is his older brother which he just found out he had one. This brother got everything in the inheritance left by their father (according to the will) and nothing was left for Ikuro.

Norihiko, this stupid fucker was a long time servant in the Tōma's house even before Ikuro was born. I don't know what's his deal but it seems like he also slept with Ikuro's mother before, and now he groomed Ikuro up to adulthood to be his "toilet" and plaything.

Norihiko basically manipulates Ikuro since Ikuro has somewhat experiences a not so happy childhood; traumatized by his mother so he is afraid of women, hates his family(and his brother more so), and he has daddy issues (like mental or something because of what he witnessed when he was a child).

So flashback to what really happened to Ranzō, so far what the story showed us was that Ranzō loved his brother Ikuro even before and that's the last memory he has before he went ill. He was choked to dead by Ikuro's mother (because of jealousy), but miraculously survived but his mentality remained as a child since then. He was unable to speak as he was locked up by the father after the incident (not sure if he was also molested by the father ╮(￣▽￣)╭ ).
And for sure this is the reason why the father's will was to only give the inheritance to Ranzo and his children and nothing for Ikuro.

Now we are just waiting what will happen as Sachiko (Ikuro's arranged wife) decides to teach Ranzō to speak so he can aid Ikuro; bc as we can see Norihiko is basically ruining the Tōma family and having old boomers rape Ikuro and arrange everything in the company(not sure if he has grudges or what).

This is the first time that I was actually disgusted by the seme like dafuc dude just die and let Ikuro live happily ever after with Ranzō and Sachiko. #－.－)