Wow, I'm really touched by this one because whoever you are, anon, I can tell you've really read a lot about him. I can't thank you enough for paying attention to him that way, know that it means a whole lot to me.

As for the answer itself, I think that a huge part of it has a root in Cetus' very social nature. It's who he is to seek interactions with others, and ultimately that becomes sort of the seed for some of his developments in childhood. For example, when he's young, while he has no biological parents to speak of, he ends up becoming very attached to Doctor Beryl (my friend Chich's OC!), an Aegirian scientist on the Abyssal Hunter project. I think his relationship with her is very foundational for preserving the loving parts of his personality that may have otherwise been lost in his transformation into a soldier. She's a very motherly figure to him and that bit of very human connection in an environment that rewarded singlemindedness and obedience helped to always keep him in touch with the aspects of his personality that didn't make him a star hunter - but did make him Cetus.

There's also his interactions with Skadi in their childhoods (I've decided they probably would have been trained together since they're close in age) that sort of drive him in a different way, but I think that would probably need it's own question to explain thoroughly. (Many thoughts, lots of narrative)

But yeah, I think that's about it. Thank you so much for the question, it was a great one!