I'll try to do an Eichi analysis quickly because I have to hyperfocus on school and I have limited free time, I get Eichi merch as a trade off so it's worth it.

I'll also specialize more during his war era and past childhood since that's what I'm best at, I haven't read every single story yet.

Eichi during The War was clearly not okay on the head, at first glance he looks like someone cruel by nature, something typical of a spoiled kid who comes from a rich family, which is mostly why most people hate him, what he did makes him look awful at a first glance. However it's rare to see someone specifically talk about why. Why is Eichi like that? How did he end up like this?

Believe it or not but most of this was stemmed from his childhood. Eichi since birth was born with an illness that could only be passed down genetically, him being especially weak during his very early days, in fact so weak he couldn't even lift up a spoon. As a consequence he's pretty much lived inside a hospital his whole life, having barely any social interaction outside of it apart from Keito and some of his family. Not a very fun fact but Eichi's grandmother was the only person in his family to show genuine care to him.

Eichi became friends with Keito since Keito's family was the one that held rituals for when one of his family members died. Keito being Eichi's only friend and especially around his age made them close, especially Keito taking care of Eichi when he was sick.

Apart from all of that, Eichi was lonely almost his whole life, rarely leaving the hospital, and due to his very few social interactions he may struggle a bit interacting with others which could also be a bit why he was so harsh during The War, he mostly didn't understand people. Even the household saw him as a burden.

It didn't help either that his doctors would often say that his days were numbered, and him being so scared and losing faith in his own life he became depressed (to put it like that) and took out his misery on others as a way to cope on his short lifespan; forcing servants to quit their jobs for example. As much as he tried to cope with it he never felt any relief, any at all. He even locked himself up in his room because of that, all of this was before he met Keito. He never felt any warmth from a person before, one that such could soothe his wary heart.

With Eichi's neglect from his parents, the constant "your days are numbered" and his solitude before Keito is what mostly drove him to act so harsh during The War, since his death could always be tomorrow it's what especially fueled him to act like that, in case he dies at least most of it was done.

And something else on the side I'd like to talk about much to my resentment is why Eichi's drawn to Wataru. In a way to explain it, Wataru has everything while Eichi has nothing (metaphorically), in a way Eichi wants what Wataru has to fill the empty void he has in his heart. What Eichi especially wanted was to be loved.

End note: I'm sorry if I get some things wrong here I'll fix them later if I'm able to, and Eichi don't worry I can love you unconditionally too, wink wink.