leaving sailor wasnt an option, from percy's perspective. call it nobility, call it perversion, he probably would've rationalized her as a "patient." a project. wherever she was, she wasn't being taken care of, and he had the money, space, and knowledge to do it himself. sailor isn't like what he expected though. his project didn't go as expected. it was probably easy for him to point at a simple paternal desire to explain his attached feelings, but it was much more than that and only told a fraction of the truth, the rest was buried under conveniences. conventional behavioral therapy didn't work on sailor, and his intuition led him to treating her in a way he never had anyone before. they respond to each other each moment, through movements, expressions, they guide each other. nothing slips by her, she knows when he's lying, even if she doesn't know about what. it became apparent that ever leaving her would be the ultimate betrayal, he had to make a decision to want her. percy commits to this by deconstructing everything he's been learning for the last 20 years, and using the leftover parts to make something for the both of them
darcy-- "From Sailor's side of things, nothing felt unusual for her. It wasn't scary having a stranger pick her up and take her to his house. What was there to be afraid of? Dying, like she already wanted to? Getting hurt, like she already was? When Percy found her, she was blistered, cut up, barely wearing anything when it was so cold out, a face puffy and sore like she was sick. They barely reacted to him when he picked them up. Then when they show up to his house, they'll just look around like they aren't covered in their own blood and sweat like "Your house is so boring. sits on couch, snoops at dvds". They were perfectly comfortable like they were friends.
Then over time, when shes expecting to be left and hurt again and realizes how attached she is and how much she needs him, she breaks down, she gets worse, and Percy is wondering why.
They both have to make a decision. Faith forward, pain in the past, and let the distance between them they've been keeping naturally fall away together. Otherwise, it would've just been preventable, lonely pain. They finally had each other- why should they try and act like it could be thrown away."