There's so many things that can be said about Katekyo Hitman Reborn (KHR) - good and bad things, but mostly the latter.
Our main protagonist, Tsuna, is a rather atypical hero in comparison to what you'd find in your standard fare of shounen manga in a sense. Tsuna has little to no self-confidence and believes he amounts to nothing much at all, himself and others referring to him as "No Good Tsuna" for having no spine, poor grades, terrible luck, and a lack of athleticism as he catches basketballs with his face in P.E. Bad news for Tsuna is he's assigned a mysterious home tutor who says he's an assassin that has been hired to turn him into a suitable mafia boss - a future boss of the Vongola, the greatest mafia family in Italy. He screams and cowers in fear, always doubting himself and his abilities as the plot calls him for action.
This manga is a forgettable mess for roughly 60 chapters, making that seven volumes worth of material in which absolutely nothing of value takes place in the overall story. In this period of time, the manga mainly focuses on comedic acts and has little semblance of a plot as it goes through Tsuna's everyday life and the shenanigans that surround him as he's under Reborn's tutelage. This is all prior to the manga's push into becoming more focused on action and battles as it went, joining its Weekly Shounen Jump brethren as a full fledged battle shounen manga. The thing is, this is not smart comedy. Its over reliance on a number of running gags - Dino becoming an absolute klutz when none of his men are near him, Yamamoto having no clue as to what's going on around right in front of him, Ryohei's entire characterization as a bumbling idiot - most having to do with the apparent comical character flaws its characters possess. KHR's childish style of comedy is clearly not my cup of tea as it leaves me shaking my head at the thought anyone finding these acts humorous. The manga attempts to make you laugh by portraying its characters as buffoons. It very rarely switches up its execution and corny jokes become stale corny jokes.
A lot of times during the fights, the art is muddled by speed lines and unnecessary detail that distracts from the action, making it hard to pick out exactly what's happening at times. The artwork is often messy when out of fights as well, but less so as the manga goes on. A lot of the character designs go through a significant change. No Good Tsuna goes from a plain, lanky hero to a bishounen. That can used to define the general art shift as the male characters begin to look more and more feminine until some are even prettier than most of the female characters. There's a reason why this series is known as " fujioshi-bait."
As the story goes on and becomes more serious as it deals with actual mafia affairs, you expect this is all leading up to and building character arcs as the heroes are put into these dangerous situations. If you are indeed, you may wind up more than a little disappointed with the writing and story of KHR. Tsuna is still as reluctant as he was in the beginning of the series towards the end, whining and complaining, never taking full responsibility, outright refusing to be involved with the mafia still in the latest chapters, only rising to the occasion when one of his friend's life is endangered. His naivety is just frustrating altogether as he never really seems to learn anything in particular from his encounters that changes his character in a significant manner. His rebuttals to the Big Bads usually consist of the cliche "I'll protect my friends" line and nothing actually inspiring raises out of it because of this disconnection between the characters. Even at the end of the manga, 409 chapters in, he's fundamentally the same exact character, remaining stagnant even after all of his endeavors, and he even acknowledges it himself. So much for character development.
Characters in this series never particularly caught my attention as I felt largely indifferent towards the cast and the conflict within the story. There's a severe lack of dimensions to the characters as they possess only a handful of character traits usually with one particular exaggerated personality quirk that is meant to be a character flaw.
Chrome, the obligatory girl of the main cast, holds little to no worth at all, her ability to do nothing without the aid of her precious "Mukuro-sama" grates on the nerves. She simply serves as Mukuro's vessel, and then the series does a 180 and she is no longer useful even for that purpose. Ryohei should've remained the annoying comic relief character he was as he's what you'd call a muscle bound idiot without any charm. One who adds "To the extreme!" at the end of his sentences, an example of the exaggeration I was talking about before.
The Varia Arc is little more the than obligatory Tournament Arc that's become somewhat a stable in shounen manga. With that in mind, it's a shame to say this is KHR's at its absolute best. Xanxus wishes to stage a coup d'etat and take over the Vongola family after finding out he was adopted, meaning his bloodline restricts him from becoming the tenth boss. Of course, it has some twists, but it's still little more than a tournament arc with the gang training for its arrival.
The motivations of Byakuran don't go beyond "I'm bad and I want to kill, have some fun, and rule the world." I can't tell you how much that leaves to be desired, much of the characters' struggles aren't felt due to the complete shallowness of its antagonists - Byakuran being the main one for the longest arc. He does, in fact, have an unnecessary sob story that doesn't justify his motivations at all.
A thing that can be praised in KHR is its battle system, utilizing what is called Dying Will Flames with varying properties to differentiate fighters and also create a battle system with some complexity. They are used in conjunction with various types of weapons and fights have the potential to be very interesting. Illusions, animal based weapons, cloaks, gloves, rings, and so forth.
Execution in the manga, however, often leaves you puzzled. For example, in the finale battle of the Future Arc, Tsuna finally gets his anticipated show down with Byakuran after an agonizing wait. Much of the good guys are down for the count and it's all up to him to defeat him. Byakuran overpowers him rather easily and all seems lost. He then gets some power up via some awkward form of emotional catharsis, recognizing how his friends are there for him and everyone has helped him in some way, and deus ex machina involving the rings.
He's ready to go, fighting Byakuran on equal terms. No, but wait, Byakuran was only using 80 percent of his power so far. But, hey, Tsuna's only been using 50 percent of his new found power. By the books dialogue in a shounen battle manga without a twinge of humor or twist of any kind.
Riddled with cliches, deus ex machina, and sloppy writing is my diagnosis of this manga.
