The feeling of isolation is pretty core part two of my characters Rose and Alex.

I haven't reached the point in the story to explore that, but Rose always felt isolated from other people because she had a hard time socialising and making friends. The need for and the difficulty obtaining human connection is core part of her motivation for action later down the line.

Alex struggled with the same problem in addition to being pressured to act a certain way by his parents, leaving him with the impression that for most of his life his parents hated him.
Major spoiler for Alex motivations
After he became Duke his isolation became worse due to his title. The difference in class made it difficult to interact with people normally, and it wasn’t helped by the perceived need to drive people away from him to prevent them from discovering all the skeletons in his closet, and compromising his right to rule.
I realise this might also be a good moment to start talking about the other thing I’m writing and that I have yet published. (hopefully, I can fix that soon)

Ariel, No. 32, not only felt left out of humanity, she was denied her humanity. Spending the first few years of her being on the receiving end of inhumane experiments due to her psychic power. Followed by a decade of being raised and trained to be a living weapon.
But she still wants to fit in some way: “General Verbast made a deal with us. He made it clear we could never be treated as normal people, especially not after No.19’s rampage. We were too much of a risk to the nation. He offered us an alternative: as soldiers we could be treated like everyone else in uniform and appease the politicians.”
And it is the part of the reason she continues to serve the nation that enslaved her.