Sep 21, 2021
Okay, here comes the praise. The cover is great, the title is great (though a bit misleading since he's not alone, lol), the synopsis is okay but needs a correction.
'However, in order to find Death'. Put a comma after the word 'Death'.
As for the story, you have a good grasp of entertaining dialogue, the physical movement of characters, and even the rhythm of plot points and the mood that goes along with them. When Eric arrived, you transitioned nicely from a light hearted scene to a tragic scene. Sabishi going berserk, lying about the demons killing the humans, and his willingness to coldly abandon Eric are great traits to set him apart from other wandering samurai protagnosts. He seems like a flawed, interesting character. What I'm trying to say is, you have natural potential as a writer, something not everyone has.
Here comes the criticism. Let's see if I can't hammer you into a better artist. I found Jonah annoying. He reminds me of Puck or Isidro from Berserk. Just an annoying little twerp who's meant to be the comedic relief in contrast to the stoic protagonist but ends up just being aggravating. I think your first instinct about writing a story about a lone swordsman was correct. He can meet people like Eric, but either he abandons them or they abandon him. Come to think of it, this is very much in the tradition of the wandering samurai. So, yeah, kill Jonah. 
My other big gripe was that you didn't describe the demons at all. Big mistake. Fiction like this RELIES on the monsters being interesting. I remember reading a Shonen Jump manga called Black Torch that got cancelled after 19 chapters; its human designs were fine, but its monster designs were bad, and THAT'S what killed it. You don't have to be a Kentaro Miura of Berserk fame-level monster designer, just give your monsters SOMETHING that sets them apart. For example, Koyoharu Gotouge, the mangaka of Demon Slayer, just makes her demons humans with fangs, long nails, and cool tattoos, but that's enough. You could give the demons animal heads, make them different colours to mark how powerful they are (red, blue, yellow, green, etc.), give them the traits of yokai, etc. Use your imagination.
Lastly, grammar. Break your writing into paragraphs, with each paragraph focusing on a different subject. Don't use speaking verbs like mutters, shouts, whispers after the dialogue has already been said; that's retroactively trying to place a tone on something that's already been said neutrally in your head. Don't capitalise the he or she after dialogue.
Here are some useful links to make you a better writer:
The Elements of Style (concerned with good grammar and sentence structure)
http://www.jlakes.org/ch/web/The-elements-of-style.pdf]
The Hero with a Thousand Faces (concerned with story)
http://www.rosenfels.org/Joseph%20Campbell%20-%20The%20Hero%20With%20A%20Thousand%20Faces,%20Commemorative%20Edition%20%282004%29.pdf
Glossary of grammar terminology
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/244216/English_Glossary.pdf
Sentence structure and types of sentences
https://academicguides.waldenu.edu/writingcenter/grammar/sentencestructure
Conjunctions
https://academicguides.waldenu.edu/writingcenter/grammar/conjunctions
Hyphens
https://www.grammarbook.com/punctuation/hyphens.asp
Peace! 
