Here's your royal road to the official Cartoon Network explanation of the shows (http://www.cartoonnetwork.com/watch/tv_shows/billymandy/), complete with character profiles, including Nergal, whom Mandy often describes as "Grim's arch-enemy".
It is worth noting that he won Billy's Aunt Sis's heart almost in spite of himself--his desperate campaign to win her over was going nowhere fast, when Billy, desperate to save his aunt, dropped a hornet's nest on Nergal. Aunt Sis mistook Nergal's desperate gyrations as hot Saturday Night Fever dance moves and was instantly smitten. Of course, this means that "Son of Nergal" is Billy's cousin...Given the Sumerian angle, I guess this means Aunt Sis = Ereshkigal--funny, she doesn't look Sumerian!
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It is worth noting that he won Billy's Aunt Sis's heart almost in spite of himself--his desperate campaign to win her over was going nowhere fast, when Billy, desperate to save his aunt, dropped a hornet's nest on Nergal. Aunt Sis mistook Nergal's desperate gyrations as hot Saturday Night Fever dance moves and was instantly smitten. Of course, this means that "Son of Nergal" is Billy's cousin...Given the Sumerian angle, I guess this means Aunt Sis = Ereshkigal--funny, she doesn't look Sumerian!