SO I AM SEVERAL WEEKS LATE.
SUE ME.
now for some tl;dr with spoilers
1. The Beginning
From what Oji tells us; the world was once in a perpetual state of utopia. No one died! I'm not sure how the humans pulled that off, but the robots were robots and they all co-existed in harmony (which makes sense to how Jin was raised by his Master). Except the Braiking Boss didn't quite like that. He believed robots to be superior, to surpass humans in every way. So he raised a rebellion, pretty much made the human race an endangered species and ruled the world with an iron fist. Was there opposition? I... don't think so. Not from the human side anyway.
And there was Luna. I'm not sure why the human scientists created Luna and what her real purpose in tangent to the Braiking Boss is (that's never explained) but her power was to grant "death" to robots and humans alike. I don't think she actually killed them, but she granted them a real lifespan to live out their lives and then die. Except the Braiking Boss didn't quite like that and saw it as a threat to his power. So he ordered Casshern (and as backup, I believe, Dio) to kill Luna.
Casshern got there first. And succeeded. Except it didn't quite go the way it was planned. I think out of all the robots in the series, the only ones who are a hybrid of any kind are Casshern, Luna and Ringo (again, I'm not sure about Dio since Dio and Casshern were built at the same time and Dio says they were exactly alike. The way he died seems to suggest that perhaps Dio had the more robotic parts to him while retaining a personality while Casshern kept the more "human" aspects and was a plain killing machine).
ANYWAY, killing Luna had an effect since (and I quote) "released the death in my heart onto the world". The clash of two beings, one who granted death but was the personification of life while the other was a killing machine, completely dead to the world -- their blood mixed (and Luna's cells, I believe, are unique, so I think Casshern is now a bit of Luna's robotic/unique makeup) and released them from death (aka immortal). At the same time, everyone who was immortal began to feel the effects of the Ruin.
That is Casshern's past in a nutshell. Now with a bit of Luna, he got a personality (which I think is why he lost his memories. A form of coping mechanism from that new overload of emotional information) and he's got a whole lot of problems with the world and himself.
2. Casshern in the End.
I think I made it abundantly clear to a lot of people how I felt about the end of the show, but here's what happened.
Luna was both destroying robots as well as resuscitating the ones she wanted to save. She had turned on herself, becoming petty and spoilt. Casshern was confused at the end result since he's unsure what good Luna is doing (I think it wars on the fact that she is still saving people unlike himself and yet, there was a strong sense of wrongness in the way the people she saved lived; aka they lived without care, without feeling, without a sense of life and the importance of living residing in them, unlike everyone else he has ever met). Towards the end, when Dio rescues him from Leda and demands Casshern fight him ("You're my joy of living!"), is when Casshern really understands why he felt so conflicted about Luna to begin with. People just weren't living! There wasn't any worth in their lives. And to Casshern, who can understand how sad and how fleeting life is, this really hits him hard. It makes him feel worse when he's the only one who can never truly experience what it's like to live and die, concepts he finds that are interlinked.
He explains this to Luna, who doesn't get it. And leaves with Ringo, Oji and Lyuze. Happy-ish ending right?
Wrong.
He does have a semi-happy life, though it's pretty brief. Oji dies, succumbing to the Ruin and not long after, Lyuze follows him. Lyuze's death hits him the worst out of any thing I've seen so far since Lyuze has been... pretty consistent! She's always been by his side, someway or another. And her last words probably made him cry a little inside.
"I'm happy to say this. I don't want to die"
Despite all of Casshern's conflicts with Luna and her "eternal kingdom, eternal lives, death is bad", he doesn't want to deprive people of... wanting to live, or living in general. When he meets Luna for the last time, he tells her that she's free to keep healing people. But if anyone forgot about death, he'll come back and remind them. And then he leaves.
MY THOUGHTS; I'm... pretty sure he just goes around wandering the world on his lonesome after that and resists any kind of contact with people. It'll defeat the purpose if he was to meet/talk to people again after this point. He knows what he is, now. The incarnation of death to a world that needs that reminder. I assume that Luna manages to heal the world (or... some of it) and Casshern just... keeps an eye out. He's been through a sense of loss and while that's painful for him, it keeps him going on this singular purpose he has in mind.
3. Canon-updating.
I am admittedly waiting for a Dio first before I canon-update. And if I do, it'll definitely after episode 23 instead of episode 24 because post-24, he's not playable. He would be... I'm sad to say, even more depressing and less likely to interact and want to be with people, if he has considered his true role, the way to atone his sins, as Death. 23 on the other hand, would be a better point to play from, since he has accepted that he can't really live, but at least he's made a choice instead of being confused on what to do/how to help the world. I probably will not canon-update for a while if I do not get a Dio soon. If I do, I will probably sit and hash out details first since Dio moves from "That guy who hates him" to "The guy who taught me what life is".