Mob Psycho 100 being a quite complete manga with a good ending, a sequel/spinoff wasn't necessarily needed, but it sure as hell is very much appreciated when it comes in the form of REIGEN.
Characters & Story:
While REIGEN gets rid of the original main character —Mob— for most of its duration, it's also significantly less Reigen-focused one might expect from the title. Instead, it breathes a whole new life into Tome (who was previously a pretty amusing side character) and her developing relationships with the employees of Spirits & Such, which of course include Reigen, but also Serizawa and Dimple, who all bounce off
one another in very amusing ways.
Following Tome's descent into the legitimately paranormal world, the manga is surprisingly both more horrific and more comedic than MP100 was. You can tell ONE has learned a lot from his previous manga and he does not hesitate to show it. The interactions feel organic, Tome's struggles both funny and relatable, and the paranormal looks way scarier than it ever did before. In just 7 chapters, Tome is promoted from funny-but-forgettable side character to a full-fledged main character the reader can really care about, the chemistry she has with Reigen, Serizawa and Dimple probably helping a lot in that regard.
Said people also get some valuable development, so even if you don't think you can get invested into Tome, there's still a lot to get from this volume.
This manga not only benefits from ONE's huge artistic improvement gained throughout the course of MP100, but also from his assistants who do quite a bit more work than they did in MP100. While ONE has clearly mastered horror and draws amazing badass shots and terrifying monsters, his assistants do wonders drawing some of the side characters and the occasional Tome. ONE's excellent character designs coupled with his assistants' appealing line work and experience mix very well. All in all, it's one step quite ahead of MP100 in terms of artwork.
This is an excellent sequel/spinoff to Mob Psycho 100, so if you're a fan this is definitely a must-read.