Who are ninjas? Elusive killers, wrapped in all black? Spies?

Not really. The word "ninja" literally means "hidden warrior", "infiltrator".

The word "ninja" as a reading option

nepornn中oB 忍者 cTano uMpoKo

used only in the mid-20th century, historically these warriors were called shinobi or shinobi no mono. They began to appear in the 13th century: with the growth of internecine wars over territories, opponents needed “intelligence” that could not be obtained legally.