
You got it except they didnt help him bc he’s amnesiac nor a kid. He didnt talk at all in the first few years on top of not being a big talker at all so theres no way they found out about his memories before they started to help him and its the same with him not speaking their language. They helped him bc he wasnt like the others. He was naive and trusting which wouldve gotten himself killed had he not known how to defend himself. Which he did. That and he was the perfect listener

He really wasnt an underdog. He was on the brink of death from surviving a plane crash with a head injury. which doesnt count as an underdog since he wasnt naturally weak but injured. But he eventually got better and whilst that was happening he was a sponge and was absorbing everything everyone else was doing. I didnt say you were completely wrong but you're point doesnt stand because they werent helping him because he was an "underdog" because he wasnt one. He literally had the most skill out of them all which is why he was made 001. He didnt become 001 out of nowhere. They helped him because he was the only one that wasnt out to get them and wasnt egotistical. He wasnt aiming to kill them nor was he trying to fight them. He just sat there, saying nothing and doing nothing unless he was forced to. Unlike everyone else who were going out of their way to kill one another and trick eachother into killing the other.

He was small, weak and injured when they met him which is.... An underdog. It literally just means "someone who has nearly no chances of winning" and that was him at the start. That's also up to you how you'll interpret why they helped him out, but pretty sure he really was an underdog at least up to the point where he healed enough to fight back decently. Whether that only took weeks or a year, he still was like a wet cat to them for that short period lol
Ohhh now i get it. Ijin is probably the youngest among the numbers, with maybe 032 the only one around the same age. Ijin also had amnesia and spoke an entirely different language. Humans are naturally drawn to underdogs, so each of them started giving ijin tiny help here and there (giving him extra food, teaching him how to fight, etc). He also had no ego, but became strong enough to keep others in line. And most of all, 001 *cared*. He doesn't say a lot, but he delivers, both to their higher ups, and to the numbers themselves. If, say, 005 became 001, she's snarky and expressive enough that someone might act out. 002 is that, second best -- because he can be overbearing too. 008 is on the softer side a little too much to hold the highly individualistic numbers in line.
Having ijin as 001 make them all want to work together to keep this tiny ass amnesiac kid somewhat safe (to some level). And that was VERY evident when Ijin betrayed them to escape. 25 of them LET him go. Because they wanted to keep him safe. And safe was away from them.
And when 001 was gone, the numbers started to operate individually. To prevent them from planning a rebellion, but also because they don't work as good as they used to anymore. But they found out Ijin is alive and suddenly a rebellion actually happens lol. They united again because 001 is "back"
Also funny how 003 is a direct contrast to ijin -- she looks soft, promises a lot, but doesn't really follow through. While Ijin has the intimidation factor down, doesn't say much, but always follows through his unspoken promises.