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.

Personality
As seen in the Devilgram Battle for the Bread!, Thirteen isn't the strongest singer. Asmodeus comments that she's "completely tone-deaf."

Likes

 * Thirteen prefers small dogs over large ones because she thinks they're cuter, as stated in the Devilgram Battle for the Bread!.

Story
Note: this section may include spoilers

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.

Relationships
Note: this section may include spoilers

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