Loki: Character looks good, accepted. 

Silver: I'm afraid I don't have any decent semblance ideas. It is possible he doesn't know it, I mean, look at Jean. The fact that he got accepted into Beacon makes that seem unlikely, though. (I wouldn't be opposed to it if you can find some way to incorporate it into backstory.)

Merman: Character looks pretty good, except the weapon. As far as lore goes weapons use Dust-based ammunition, and even if solar-powered weaponry was a relatively new discovery it probably wouldn't be in the hands of a kid, child of a genius inventor or not. So I'd say change the weapon a bit and you're good to go.