Does The Grim Reaper work for The Devil or for God?

Well the popular depiction of the Grim Reaper is a creation of traditions linked to non- Christian origins.. The reapers found in the Bible are not like the grim reaper of tradition..

(Revelation 14:14-20) "And I looked, and behold a white cloud, and upon the cloud one sat like unto the Son of man, having on his head a golden crown, and in his hand a sharp sickle. {15} And another angel came out of the temple, crying with a loud voice to him that sat on the cloud, Thrust in thy sickle, and reap: for the time is come for thee to reap; for the harvest of the earth is ripe. {16} And he that sat on the cloud thrust in his sickle on the earth; and the earth was reaped. {17} And another angel came out of the temple which is in heaven, he also having a sharp sickle. {18} And another angel came out from the altar, which had power over fire; and cried with a loud cry to him that had the sharp sickle, saying, Thrust in thy sharp sickle, and gather the clusters of the vine of the earth; for her grapes are fully ripe. {19} And the angel thrust in his sickle into the earth, and gathered the vine of the earth, and cast it into the great winepress of the wrath of God. {20} And the winepress was trodden without the city, and blood came out of the winepress, even unto the horse bridles, by the space of a thousand and six hundred furlongs."

So two reapers.. You do not want to be reaped by the second reaper.. You want to be among those who are reaped by the first reaper..

The grim reaper is older than christianity and while he is not part of christian mythology he survived and is still recognized by christians

So for neither

I remember reading a series in which Death was an office. It functioned as long as there was someone who held the title. The same went for other universal mechanisms. Fate, Time, War, Nature, Good and Evil.
The office holders were humans and would be replaced from time to time to keep everything up to date.
Death can’t be evil or good. It would have to function for both.

Actually, for Billy and Mandy.

The "Grim reaper" is a euphemism for Death. Either tĥier times-up on the biological clock or someone had a very bad oopsy

IF I believed for a moment that your God existed and was in charge of everything, I would say the Grim Reaper works for him. There's also evidence in the Bible that the devil works for him.