Quite a nice game, I really enjoyed playing it. Pixel art is very well done and gameplay, while simple, keeps being fresh enough to keep player attention, mainly in form of creative boss fights. Really enjoyed the soundtrack too.

Overall I think it's a must have for any Hololive fan and generally a nice short game for anyone else looking for some Megaman inspired action.

Spoiler free part:
Flare Nuinui Quest is a really good platformer with a few minor flaws, some rooted in the game itself, some rooted in me personally. I don't remember if it was the Dev themselves or the fandom but this game was advertised as a MegaMan like and in that case it does deliver partly. Structurally it is a linear platformer, what it does have in common with MegaMan is the "weapon absorbtion" mechanic in that you gain a weapon after every stage corresponding to the endboss of said stage. However, this is where the similarities with Megaman end imo. Other weapons have no ammo system with your standard weapon having unlimited ammo, making strategic use of the better weapons neccesary, weapons don't deal more damage based on a certain bosses weakness (for the most part, there are some exceptions). At least that is my experience, I've tried all weapons against many bosses and they didn't seem make a difference per boss.
However, putting the comparison with Megaman aside, this game is an excellent game for Hololive fans. A lot of Holosongs get remixed, the stages are very varied and interesting, platforming with Flare feels pretty good and the difficulty curve feels very fair. The few flaws I could mention here are how sometimes tiles blend too much into the background so that it gets hard to see where you can step and jump and where not and completing the game is a pretty big pain imo.

Spoiler part:
This is because after finishing the game with the true ending you unlock Noel, Pekora and Marine mode. Noel mode is basically the normal game only that you can ONLY play as Noel, who you unlock in the regular Flare quest after stage 4. It differs in a way that a) she has only a Melee attack b) she has no double jump and c) her charged attack deals more damage than any of Flares guns. However, there are passages that are very clearly designed with Flare in mind. Some jumps are already tricky WITH the double jump, especially when there are enemies who can just shoot you off a platform. So Noel mode felt very rough to complete. Pekora mode has you gather crystals and rescue her fans as a side objective. She has her signature rocket launcher that seeks targets and lets you rocket jump to platform. I felt Pekora was very clunky to play as since her rocket launcher als pushes her back a bit if you shoot forward and her movement speed and jump arc are lower than that of any other character. Peko mode was overall the weakest campaign for me personally. Marine mode on the other hand was amazing. Here the objective shifts a bit from regular gameplay. You still have your cutlass as a melee weapon but you also have your grappling hook that can either damage enemies it hits or, and this is the brilliant part, lodge into ceilings to swing and find your way through the stages that way. Also each stage comes with 3 objectives which give you one gold coin each. These objectives are always to finish a stage in a certain time, take a certain amount of damage max and the third one can vary iirc. Some of these missions had me curse out a lot, especially the last ones, es they felt sometimes very cheap not less in part because I would angle my control stick diagonally upwards to shoot the hook in that direction but it would either register as only right or only up. This and the final boss did muddle my excitement for Marines quest a bit but I realize that might just be my controller being the issue.

Overall I'd say if you are a Holofan and you have some kind of spot in your heart for Platformers, get this game. Even if you are not a Holo fan it is still a very good game, especially for such a low-low price. If you are not a completionist even better, otherwise you might find some frustrations here and there.

I'd give it an 8/10.