Thirteen

"'The owner of the reaper's cave, Thirteen has been acquainted with Solomon for over a thousand years, and often competes with him in games for his soul. She always names the traps she makes for fun, and you can say the naming is rather unique.'"

Thirteen is first mentioned as simply "the reaper" in Lesson 35-5. She is a powerful entity that watches over the "life candles" that represent the souls of humans, demons, and angels.

Story
Note: this section may include spoilers

Background
Solomon has stated that Thirteen is very aggressive towards him, though this is because she once ate his cooking.

Main Story and Events
In Lesson 38, "the reaper" became angry at Beelzebub for accidentally breaking Lucifer's cursed record "Hymn of the Reaper." In retaliation, she caused Beelzebub's life candle to burn significantly faster and almost extinguish.