It could imply that even a humble or unassuming swordsman (assuming 'kensei' means swordsman) is still the strongest in the context of the web novel. Maybe it's about a character who doesn't seem extraordinary at first glance but has hidden, overwhelming power.
It can introduce an element of surprise. The plot can revolve around the other characters gradually realizing the true strength of the 'jimi na kensei'.
I'm not sure exactly as I haven't read it. But generally, a light novel often involves a unique story, perhaps in this one it could be about a vampire (kyuuketsuki) and their daily life including an afternoon nap (ohirune).
The following is a classic quote from Jimmy:
1 A garland
a Garland
Another small circle
around me
around me
A Dream
a dream
a beautiful dream
in the dark
in the dark
3 A Cup
a cup
Fell on the table
shattered
shattered
A Smile
a smile
Another small mouth
towards me
towards me
A Melody
a melody
Another small section
around it
around it
One Eye
one eye
A bright eye
look at me
look at me
A Story
one story
a moving story
in my heart
in my heart
8 A Star
a star
Another little bright spot
in the sky
in the sky
9. One Road
a road
A winding road
leading to my
leading to my
A World
a world
a beautiful world
in my heart
in my heart
It could mean a lot of things. Maybe it's a name of a special being in the fanfic's version of the Naruto world. It might be an elf - like creature that has some kind of power that can't be tamed or understood, which is what 'yaserarenai' could imply.
Perhaps the main plot is that Kobayashi San's loved one is kidnapped in River City and a ransom is demanded. Kobayashi San has to stay calm while trying to gather the ransom money and figure out a way to rescue the hostage without getting caught by the kidnappers.
The title 'Stay Cool Kobayashi San A River City Ransom Story' gives the impression of a story full of suspense. It could be that Kobayashi San is an ordinary person thrust into an extraordinary situation in River City. The ransom aspect implies that there is some sort of threat or blackmail involved. Maybe Kobayashi San has to use their wits and courage to resolve the situation while staying calm. It might also explore the culture and environment of River City as part of the backdrop to this intense situation.
Yes, it can be used. You could incorporate 'elf san wa yaserarenai' as a unique concept in a Naruto fanfic. For example, if you have a character in the Naruto world who is from a different dimension or has a strange power, you can name it 'elf san wa yaserarenai' and build a story around how it affects the Naruto universe, like how the characters interact with this new element and how it changes the power dynamics.
Some local libraries that have a good collection of international or Japanese literature might also have it. You can also search in some anime or manga - related forums. Sometimes fans share information about where to find rare or not - so - popular light novels like this one.
I'm not sure specifically as there could be many 'onii chan dakedo' light novels. It might be about the relationship between a younger character and an older brother - like (onii chan) figure, perhaps exploring themes like family, friendship or some sort of coming - of - age story within a light novel setting.