You do a really excellent Severus, and your portrayal of Regulus is fun and convincing, too. I like the way they mature through the years and how Regulus manages to head off every one of Severus's suspicious insecurities and doubts and sneering defenses. I also like how plausibly the fic fits into the cracks of canon.
Beneath his arrogant attitude were a calculating mind and layers of subtlety that his idiot brother could never hope to possess. The attraction is perfectly clear!
I do love the fact that JKR leaves so many canon bodies undiscovered -- it means we can have believable stories like this one.