One of the very first things that I wanted to know before I read this amazing Novel was " Whats THE DIFFERENCE BETWEEN THE NOVEL AND THE MANGA " Then the question " WHATS THE DIFFERENCE BETWEEN THE WEBNOVEL AND THE LIGHT NOVEL", If you want to know just scroll down to the lower half of the review.
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Story and characters : the story about an average Otaku who is drawn to a world and becomes a hero and then is betrayed and becomes a hatefull cold amazing human being , One of the best traits about this novel/manga is that the protagonist is a character with so many human flaws , you sense he is a bastard from earth ,Not an imaginary character who is so Full of shit, the rest of the characters are unique in their own way breaking the forth wall sometimes , from a swindling merchant to a hero with big family .
The anime references are massive in this novel, specially those of the fist of the north star.Characters break into those with a natural flow , the writer has an amazing ability , and i cant wait for his next work.
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For story development , the story in the LN/MANGA is going smoothly and it is getting a good touch on the editing , the character development as an ending is so great with a really satisfying ending, you will never regret reading till the end .
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Art: as I read the webnovel, the art is acceptable although it isnt the best art but it was good.
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Enjoyment : The enjoyment in this novel is topnotch , it is really funny and energetic , something that i couldnt find in many mangas/novels and the best thing is that there are many side stories so the tale will never end such as that of christmas of the shield and the valentine version ,
most of my enjoyment came from FIROO-TANN !!! AND MOTOYASO , amazing characters, with amazing reactions, it was really fun to read and interact with each action , The romance only at the end, not ruining the story with uncalled for barfing romance , filled with heartwarming =chapters .
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should you waste your time reading this?
Yes you definitely should , its worth to read to the last chapter and never give up in the middle.
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Now answering your questions , the difference between the manga and LN ?
NONE
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But!! the LN and Web Novel is HUGE!!
Starting with this , The web novel is like the unattended side of the story filled with many events and things that you think is a waste of time, its funny but you will get to say what am I doing around vol 7-vol 12 .

The WN is full of thorns but reading it to the end is worth it as it is fully translated on websites and easy to get,the story if finished and its one hell of an ending , The ending in LN and Spine of story is almost the same , so it is a good read.
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FOR THE LIGHT NOVEL: (Spoilers alert)
Its currently on vol 6 JP and vol4 ENG, The story branches around the vol 2 in the LN AND MANGA as Naofumi goes across the borders to meet Raphs previous owner and get kiel (Loinclothed dog ) freed , which didnt happen in the WN as he bought Kiel from the slave dealer, Meeting FItoria the queen of filorials there ,who forces them to reconcile with the heroes which did not happen in the WN as She just tests Firos growth as a FILORIAL QUEEN.
I found info about meeting a vassal hero ,one of the seven heroes in cal mira island around vol4 which did not happen in The WN.
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SIMPLY: the web novel had many flaws such as not introducing the seven heroes earlier or not even giving info on them until later volumes, they needed to introduce new characters around vol4 and that change was made in the LN , Naofumi Never went to silt wilt or another demi human land (except zeltbur ) which was frustrating for me and looked like flaw for me ,After saying he would visit so many times, I hope they change that in the LN as they are solving so many flaws.
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In the end Thank you for reading atleast one of my words as I am but an amateur reviewer who enjoyed this Novel and still on the manga ,In the end I would like to say Fueeee!!