Re: question
I'd be all for that! And aw, Steve. Him and Constance are super cute.
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If by "secret" you means his being a demon, then yes, under the right circumstances he'd absolutely share it. Jamie already all but told him she knows he's not normal, and Yuuki flat-out said she can tell he's not human, so he already realises he might not have much of a choice. In Yuuki's case it's even shaping up to be a bonding moment! Poor Yuuki, being mistrusted because people think you're a monster, he completely gets it... Sebastian's not the type to want to wrap every random person around his finger for no reason, but if for whatever reason he'd want to endear himself to any of the people in Kiseki with a "I'm a dangerous monster ):" thing going on, that's the angle he'd spin. He's pragmatic, though, and will tweak it to whatever person he's sharing it with.
If you mean "will he share that he eats souls and kills without remorse", then... he'd much prefer not to. But he ran into one of Porthos' feather memories the other day, so he's realising it might come out regardless of his intentions. If it does, he'd probably try for "but I have to eat something right ):". Again, he's pragmatic, and if he met a person who'd be fine with his lack of morals, with whom admitting to it would benefit him, he'd be okay with doing so. The only person currently in the game I can see that happening with eventually would be Jamie, but it's not for certain and it definitely won't be right away.
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Do Sherlock and Steve interact off-screen? If so, what do they think of one another?