Thanatos

Thanatos is the strongest Grim Reaper and one of the oldest.

Appearance
Thanatos wears a skull mask and an exquisitely decorated cloak, the standard of all Grim Reapers.

Personality
Thanatos is calm and reserved, even giving his opponents respect when he complimented the transportation magic circle technique used against Loki, used on him, as being quite a clever technique. When his comrade Pluto was defeated by Vali, he commented that Pluto shouldn't have felt dissatisfied with touching the power of a Heavenly Dragon, showing his small fascination with fighting powerful opponents. He also has a rebellious side because as mentioned by Orcus, he hardly follows Hades' orders and often takes actions against Hades’s will.

History
Thanatos was born from the union between the Greek primordial sibling gods of darkness, Erebus and Nyx, sometime prior to the series.

Red Dragon Emperor of the Blazing Truth × White Dragon Emperor of the Morning Star: The True Dragon(s) of the Kuoh Academy
In Volume 24, Thanatos sent his subordinates to capture Koneko to obtain her clip which contains research data on creating Super Devils, to try and prevent the data from falling into Hades' hands. He officially appeared himself later on above Fafnir Stadium while the match between Team Rias Gremory and Team White Dragon Emperor of the Morning Star was going on, due to his subordinates attempts to interrupt this match having been thwarted. After he appeared above the stadium, he was transported to a wilderness area a far distance away, trapped inside a barrier erected by Rossweisse and Grayfia, engaging in a fight with Issei who was waiting for him. During the fight, Thanatos was outmatching Issei until Issei was aided by his grandfather Juzo, Guatama Buddha, and Guanyin, where Thanatos was finally defeated by Issei's Nyuutron Beam Cannon.

Powers & Abilities
Master Scythe Wielder: Thanatos is very skillful at wielding his scythe.

Immense Strength: Thanatos is regarded as the strongest Grim Reaper among every single Ultimate-Class Grim Reaper.

Immense Speed: Thanatos is also extremely fast, being able to create after images of himself and surpass Issei's speed in his Pseudo-Diabolos Dragon God form where he could seal Issei's cannons within a matter of seconds.

Equipment
Scythe: As the strongest Grim Reaper, Thanatos' scythe can wound his opponents and shorten their lifespan with a special type of curse which is much stronger than that of the other Grim Reapers, especially Pluto's.

Trivia

 * In Greek mythology, Thanatos is the personification of "Death".