Mon is a fallen angel and as such, works by executing ponies condemned to hell. Their is preferred form of execution is basically to drive their victims to insanity with; apparitions, accidents or tragedies. Let's say They does the "dirty work".

They lives in a cemetery, more specifically, an abandoned house, which they cleaned and tidied up making it quite homey despite living alone. They tends to wander around towns or cities and chat with lonely ponies to give them advice or fool around.

If you meet them for the first time you would think they is rude, bitter or even boring, but when they gets to know you, they will show their true self.

Loves to make jokes of all kinds and at any time, They tends to play with sarcasm or improvisation, so much so that They seems to be someone silly. They is very bad at showing affection, so they opts for teasing or mildly hurting instead. basically, a closet tsundere

Gossip

Loves to know the lives of others, but not personally, he wants to be more of a "spectator".

Stella

Mon's pastime is to tease Stella, she has little patience and does not identify sarcasm well, she is the target of Mon's silly jokes.

Uncle

Mon spends his time spoiling and spoiling Araki's son, Gun, and when he starts to cry, he simply gives him back to his father.

CATS

loves cats, collects pictures of them in an album and can talk about them for hours.

A long time ago, in Silver City, a sweet girl was born with a great future as a protective angel, since she would inherit her parents' job. But as she grew up she was acquiring a rebellious character and not at all obedient to her superiors, feeling more comfortable with demons than with her own kind; developing a short courtship with Araki (currently, her best friend), the two were making disasters and practical jokes to superior angels, they played a Russian roulette.

It was all fun until they upset the wrong person; Magnar, a very prestigious seraphim with zero sense of humor. as a seraphim, he had power over almost everyone, and enraged he gave them a somewhat harsh punishment for a couple of teenagers; to Araki he denied the entrance to any territory of silver (land of origin of angels), and for poor Ĭ̵̩͈͕͇͎͛̓̊l̷͚̥̳͊̽̔̄̇u̶͎͎̫͌̾̄͐m̴͉̒̆̂ͅì̶͙̠̪̥̽̽͋̈ṉ̷̜͈͙̩́͂ȃ̴̮̥͚͖̩̌̑̆̈, he decided to cut her wings (metaphorical), expelling definitively the angels from heaven, the worst thing you can do to an angel.

Without memory and without direction

Upon being cast out, he was stripped of his memories and a home in which to take refuge; he began to wander aimlessly through the earthly lands and it was there that he found a large and seemingly abandoned cemetery, where in the middle was a small house about to collapse, his new and perfect home. After a while, where he was basically re-discovering himself as a way to recover from his sudden amnesia, a large black star glowing with orange light appeared in front of him; Stella was her name, another fallen angel but of higher rank.

Stella, after introducing herself, went on to explain the system of the fallen angels and his new purpose: he would have to search for and execute the souls within a certain time and deliver them to her, simple, isn't it? well, Mon would understand anything but that and would cause chaos and poor Stella would have to clean up his mess.

I am who I am

Finally, with Stella's help, he found a name that fit himself; "Monotone", someone simple, boring, without many colors and above all, he doesn't give much thought to problems and loves to be like that.

And nowadays he lives quietly, without thinking about his past, in his little house in the middle of a cemetery. He met Araki again and rebuilt their friendship, and together with Stella they are a happy group of three friends (Stella doesn't know how she ended up there). A happy ending, isn't it?