Nowadays I've been watching lots of psychological anime and this anime is a great example of the perfect psychological anime. It has the perfect amount of action, adventure, psychology and just a bit of wholesomeness. ~~I'll try to make this as spoiler free as possible.~~
Plot
10/10
The anime starts when Angelos family gets killed by the Vincent Family; so he decides to get revenge... 7 years later. What I liked most about the story is how Angelo tried getting closer to Nero the Don's son so he can take out the Don instead of assassinating them one by one without revealing his identity. However, instead of getting close calls and really intense moments, we got wholesome moments (not BL) and really badass moments this of course only last for a few episodes, but I still really enjoyed it.

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Animation
8/10
The anime aired first in 2016, so the art and animation were pretty good and the art always gave the vibe the anime was looking for it could look ugly in a way when someone dies and really vibrant when it's a wholesome scene.

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Character's
8/10
Most of the characters didn't have any hidden backstories except Angelo, but a few characters developed like Corteo who became less cowardly. However, they were all solid characters with distinct personalities and all of them had different roles to keep the story going.

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Music
9/10
I personally liked the OP and ED they both represented the anime really well and sounded fantastic. The OST's were also good, played at the correct time and not repetitive. Also, the OP was made by TK so it was really addicting.

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Enjoyment
9.5/10
I really enjoyed watching this anime, It surprised me at every turn and always found a way to keep me hooked. The characters never got boring and Angelo always had a trick up his sleeve, which made the anime even more entertaining.

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Overall
9.5/10
91 days was very fun and an emotion rollercoaster full of sadness, wholesomeness, and happiness. I never felt bored while watching, and I was always waiting for what would happen next. So if you're looking for a nice and short psychological and action anime watch it it's great.