nine, regeneration & rose. ( aka jayne’s unpopular opinions. )
if we want to go with what canon suggests ( i don’t but for my example i will. ) nine regenerates from the war doctor, he is alone when he regenerates, this mixed with severe ptsd leads him to be a bit awkward in social interaction, he is mildly distrusting, and a bit arrogant though he hides his pain beneath a somewhat goofy exterior. there’s a pain behind it. at his core, he is the man who has to deal with a pain a past version of himself created, he is learning to cope with it.
when nine finds rose, he has only just started to settle into his new form. he has spent most of his time saving the universe by himself and has become somewhat lonely but doesn’t want to admit it, as he doesn’t want to risk someone else getting hurt because of him.
nine’s season is him becoming more human, it is his lesson of trusting people again, and that is why rose becomes so important to him, nine doesn’t love rose romantically because he doesn’t think he deserves to be seen that way, he doesn’t want to be seen that way, and isn’t ready for it. he falls in love with rose as a friend ( he literally flirts with others in front of her. ) these feelings change over time, he isn’t in love with her until he is. it is a realization of these feelings begins and ends with bad wolf.
he sees her, and she understands him. she has seen all of time and spaces, everything that is and will be and everything that cannot be, and she looks at him and understands how much it hurts. she comes back for him in the same way he would for her, that he did for her. she comes back dying, and he saves her because he loves her. the same way he had tried to send her home, to keep her safe because he loved her. but he didn’t deserve it, and he still wasn’t ready for it.
so while he’s regenerating, he thinks of rose, of how she is and how he has been, and he hopes. he hopes the next version of himself is better. that he will be someone who won’t hollow himself out in fear of falling. he hopes he will be brave, and handsome, and maybe not so stoic. someone who won’t run, but will whisk her away somewhere she deserves. he hopes for a romantic. a version of himself that might just deserve her.
he regenerates with those feelings and that realization and it becomes a part of ten the same way humanity is a part of him.