Villainous Noble means that they dealt with criminals, by any means (like murder/assassinations) necessary under Queen Victorias orders.
Servant's Backstories - shown in Season 1 ep 21 "His Butler, Engaging Servants"
Finnian: was experimented on (against his will) which gave him superhuman strength & a superhuman body.
Mey-Rin: former Sniper/Assassin.
Baldroy: former American Soldier/Tactician,