I have decided to post this to clear a few things up:
SSS -- Shinda Sekai Sensen [In English, it is called the Afterlife War Front]
Guild Worker -- The name says it all; someone who works at Guild! Guild is the weaponry supply for the SSS. They are located far underground. In the afterlife, anyone is able to make things they remember from life using only dirt. The Guild uses this ability to mass-produce weapons for the fight against Angel.
NPC -- Non-Player Character. Basically, it is a human in every way except for the fact that it has no soul.
GirlsDeMo -- Their full name is Girls Dead Monster, an all-female band that is composed of four members:: A lead singer [who sometimes plays the guitar as well], two guitarists, and a drummer.
More will be posted if there are more questions~
~Seika