I just decided to check the whole Jump SQ and just realised there’s a quiz section and this month was Owari no Seraph-themed, and when I checked the questions…
The last question is “Yuuichiro’s ‘family’ and Mikaela’s 'Yuu-chan,’ which one is mentioned more in the series?”
AU where everything is the same except the shield is an artifact like Mjolnir. Maybe it’s a long-lost Asgardian thing, maybe it’s some other non-Earth object. Point is, the shield is enchanted so that it only obeys the will of the wielder if their primary goal is protection. It’s just about impenetrable, can absorb any shocks, and strong enough to cut through or destroy just about anything–which would make it a perfect weapon, if anyone could figure out how to fucking use the thing. It doesn’t obey any laws of physics or movement as we know it, and SSR spends years experimenting with it until they finally give up and stick it in a crate somewhere.