Tutoring, Full Metal Panic, Black Lagoon, Queen of Arms...
The heroine is good-looking, has a black heart, and has the attributes of a black widow. Mr. Hg is a non-sexual, "occasional" seller of arms and amalgam.
The male protagonist is the violent XANXUS.
Not any thirteen-year-old boy can accept that he will have a sixteen-year-old stepmother, so XANXUS raised his gun when he saw Angel for the first time, and then... Angel became the forever number one in XANXUS's hatred list.