A head canon that refuses to leave my brain is Leon and Syo being besties. I have nothing to support this other than vibes and the fact that Leon has little enough self-preservation instincts that I think he would be the first to approach Syo after she fronts at school for the first time and is still in "Blaargh! I'm a crazy killer! Stay away!" Mode to ask "Hey, what's with the tongue thing?". Also I think he would never fully understand the whole factive thing and is genuinely a little offended that Syo doesn't think he's cute enough to kill
see i disagree imo Leon and the scissor system move in entirely different social circles and also if they did interact there's a huge chance Syo would immediately clock him as a threat to Toko's emotional wellbeing and as such refuse to interact w him/be actively hostile towards him. Syo protects Toko, and Leon isn't the safest person to be around emotion-wise. They are acquaintances/surface friends eventually but they're not close friends
also Leon would make some stupid sex joke about Syo's tongue and then he'd get clocked upside the head and with good reason