HMMMMMM wasn't sure if this applied to realistic or fantasy situations but the situation in my mind is like if they were in some sort of RPG game and were locked into certain classes/weapon types
Mimi: some form of ranged weapon because he's not very muscular. like staffs/tomes that mages used but if that's not allowed then a gun Lol. Or maybe butterfly knife for close range
Silver: whip because shes freakay like that. other than that probably just straight up fists. Like a melee fighter
Yura: Also a gun but like a sniper instead. Shes strong but she doesn't like overexerting herself so
Delta: big axe. huge fucking axe. Comically large one even. Or a big ass hammer.
Lucius: As basic as it is probably just like two swords or something. On the thinner side though if that makes sense. Think a jian sword would be cool for them to use