I wanted to let the images and the novelty of this story filter down in me for a couple of days before posting. I really enjoyed this! Percy is one of those characters who I'm always thrilled to see stories about because authors have such room to explore, and your rarepair jumped out and claimed me. :)
The characterizations for these two I felt rang so very true, from Percy's experience to Neville's downplaying of his own talents and bravery despite all he did at the height of the War. Adored how he was both tentative but also enthusiastic once he discovered Percy's keen physical attraction, and it seemed very much in character for Percy to respect Neville's boundaries, while also wishing he could move a bit faster. I see Percy differently post-DH, and your interpretation of him was in line with that.
I totally didn't see Neville's mishap in the lake coming, and once it did, I thought your creativity in it being residual Dark Magic lingering trauma from repeated Cruciatus castings to be both heart-breaking and totally believable. That Neville was stalwart about it— spot on. That he knew from some moment early on that Percy was someone he could innately trust and open up to? Seems like Neville to a T. I was grateful that Percy was so giving and careful in their intimacies, though I have high hopes that they'll be more reckless in the future.