The Fable

The Fable really reminds me of One Punch Man. First of all its both comedy. Both MCs are too OP for everyone, have incredible stamina and strength that most people don't know about. Both MC have a wannabe 'disciple'. Both MC have a stoic and calm demeanor. They speak the same and they're also both kind hearted. I think the Fable is like the seinin's version of One Punch Man. Except that they're not fighting monsters. It's just yakuza's world. I feel like there's more fighting scenes in OPM than the Fable though. The Fable has a slice of life vibe where people are just doing normal stuff but since this is about yakuza's and hitmen theres still a lot of violence in this anime. Can be disturbing for some people. I dont think the fable is for kids, Its seinin afterall read more 

the main characters of both shows are men with criminal backgrounds trying to live normal lives  

The main characters have an amazing affinity for making sense of reality in silly drawings. Both clearly traumatized from childhood. And they're both pros at what they do. 

Fable really reminds me of Trigun, with both MCs using their favorite gun, making silly faces, handling dificult situations without killing anyone. *Note that trigun is more of a fantasy comedy* 

Murders, conspiracies, criminal underworld these are the main similarities of these animes besides the emotionless protagonists 

Both shows include MCs trying to live normal lives while hiding something sinister. There are also themes of organized crime/yakuza in both; the difference is that one MC works for the yakuza while the other is trying to hide from them. They are also animated by the same studio.