Here some headcanons about the original Reaper and Reaper 2.0:
(I'll be referring to the original reaper as Korosensei sometimes btw)
I think it's pretty well accepted that 2.0 has such a high interest in flowers because it's one of the only things Reaper ever praised him for
Koro had no idea how to raise a kid. He grew up in a shitty environment and never really had a good childhood himself, so it is high likely he didn't know shit about how to treat a child
He probably tried reading a book about children once, but decided to abandon it after the first few pages and just told himself he would 'figure it out over time'
Now, it's quite known Korosensei read lots of books from all kinds of literature. What if 2.0 is the complete opposite? Reading textbooks about anatomy, science or to learn new languages? Sure, why not. But reading a book simply because of the literature? No way!
Reaper probably tried to get him to read classics like Goethe or Shakespeare and 2.0 refusing over and over again until Koro threatens to throw out his flowers.
He ended up reading them and could recall the whole plot when Koro asked him about it, but he just doesn't find any beauty in those words like Koro does. He'll use words to manipulate others into doing the things he wants, but besides that, they just don't interest him in any way.
So, we don't really know where they were born, but I like to think of 2.0 coming from western to central Europe, like France, Germany or even Great Britain. The Reaper on the other hand could be from south-east Europe, basically the balkan states.
(On tiktok I once saw a really convincing theory that the original reaper comes from Albania or Kosovo and I kinda think that as well. Credits: @thenonceller)
Imagine Korosensei being angry at 2.0 for, I dunno, being to loud or simply a little too cheeky for his liking. In one Manga panel we saw him wear sandals or slippers or something like that. Now imagine him, to make 2.0 shut up, he flips the slipper of his foot and simply throws it at the boy with a pace and strength that makes 2.0 tumble a bit at the force of impact...
Okay but to let's be real, the Reaper used to discipline 2.0 mainly by using physical punishments that could range from having to run for hours to beating the boy up while calling it "training"
Reaper threatened multiple times to kill 2.0 whenever he messed up big during a mission. When 2.0 became older Reaper would switch the threat from killing him to selling him so all the years of training and feeding/clothing the boy wouldn't be wasted.
2.0 tried to gain the Reaper's approval a lot, especially when he was younger. He'd come up with new techniques or new plans for a mission just to impress his teacher. Reaper always ignored it.
2.0 grew up in a high-society rich household, so ✨the talk✨ wasn't really something his parents would give him. Therefore he has no idea how the baby gets into the moms belly (he probably saw a pregnant woman before in his life, he knows its not the stork that brings babies) and asks Koro about it. Koro fabricates some kind of lie because he too does not want to give 2.0 ✨the talk✨ right now.
2.0 finds out a few years later from a biology book.
The boy never had a "weird phase" as a teenager, so he gets an emo phase once he's an adult and living on his own.
Koro's cooking is godlike and he taught 2.0 most of his cooking skills. (Koro justifies his incredible cooking skills with having to take on the disguise of a cook occasionally to poison his target's food. He needs good cooking skills to be convincing.)
Reaper preferred salty and spice snacks and didn't care much for sweets. With the tentacles also came his massive sweet tooth.
2.0 sometimes hyperfixated on learning certain skills or information to a point where Reaper had to physically stop him from studying and dragged him to bed.