While a bit underdeveloped plot-wise, Not Lives is a fun read with a unique take on the concept of being stuck in a game - a tough thing to do in a genre which has been done so many times. The art is stellar, the fight scenes are fun, but you won't find yourself too engrossed in the story or attached to the characters. All in all I give it a 7/10 - if you're looking for a light read in the isekai genre, it's enjoyable even if it's not deep, and worth a read.
With the amount of manga out there having the concept of being "stuck in a game", it's hard to come up with something original. And yet, Not Lives manages - with death in the game resulting in a player becoming an avatar to be controlled by another player being a pretty unique take on the genre. Besides that though, the story is unfortunately nothing special, primarily focusing on the MC and his avatar's attempts to clear the game and opting for action over plot. No explanation is ever given about what the game is, how it exists, or why it exists.
I really love the character design of Amamiya, and the fight scenes feel very crisp and easy to follow, not overly busy like some series. Most of the male characters though are not nearly as detailed or spectacular, which is a given since the MC is a bit of a self-insert character.
Most of the characters are pretty shallow. The MC himself has basically a single character trait: that he obsessively likes games and designing games. He's given no further character development beyond that, and the only things driving him to clear the game is that he wants to understand more about it and fulfill his avatar's wish purely out of altruism and curiosity. Amamiya on the other hand is initially very mysterious with little actual character development, but towards the end is more fleshed out becoming the one truly developed character in the story. Most of the villains and rivals are, unfortunately, entirely one-dimensional as well though. One thing I will say is that the author didn't descend into harem or ecchi tropes too often, and when they did, it was always tongue-in-cheek - usually with the MC commenting that he needs to take notes for when he develops a dating sim.
It's a fun series. It's not the most profound thing in the genre by far, but the fight scenes are among the best I've seen in an "isekai" manga and the pacing of reveals about the game and Amamiya's backstory are perfectly timed. You definitely won't be bored by Not Lives.