Given the title, the most prominent character should be the shounin yuusha. However, in an isekai story, there are usually other important characters too. There could be a mentor figure who helps the young merchant hero understand the ways of the isekai world. There might also be antagonists, perhaps other merchants who are jealous of his success or local villains in the isekai world who try to stop him from achieving his goals. But again, this is just a guess without having read the actual light novel.
I'm not sure exactly as I haven't read this light novel. But generally, 'isekai' often involves a character being transported to another world. Maybe the'shounin yuusha' (young merchant hero) has some sort of adventure or growth while making their way in this new world of 'isekai' that the novel is set in.
Since I haven't read this light novel specifically, it's hard to give a detailed account. However, based on the words, it seems that the story might revolve around a male protagonist (shounen yuusha). The 'isekai wo gyuujiru' part could suggest that he has some sort of extraordinary ability or mission related to the other world (isekai). Maybe he's on a journey to conquer or understand this new world, which is a common theme in light novels.
I'm not sure specifically as I haven't read this novel. It might be about a young hero ('shounen yuusha') who is involved in another world ('isekai') in some way related to 'gyuujiru' which might be a special action or concept within the story.
One could be 'Overlord'. It also has a male protagonist in an isekai setting with a lot of power - much like what the title 'shounen yuusha wa isekai wo gyuujiru' might imply. Another could be 'Konosuba'. It has a young male lead in an isekai world, though the tone is more comical compared to what we might expect from the given title.
Since we don't really know the details of 'shounen yuusha wa isekai wo gyuujiru novel', it's a bit of a stretch to recommend similar ones precisely. However, in the isekai genre, 'Re:Zero − Starting Life in Another World' is well - known. It has a young male protagonist who has to face many difficulties in an isekai world. Also, 'Konosuba: God's Blessing on This Wonderful World!' has comical elements along with the isekai setting which could potentially be somewhat similar depending on what 'shounen yuusha wa isekai wo gyuujiru novel' is really about.
The title indicates that the summoned sage is a key character. There may also be other characters like those who summoned him, or characters he meets in the isekai world such as local inhabitants, other adventurers, or even potential foes.
I don't know the main characters as I haven't read it. It could be a young hero with a special power or destiny.
As the title implies, Haru is probably a central figure. There might also be other characters in the new world that Haru meets. For example, there could be a wizard or a knight who helps or hinders Haru's journey. However, without reading the novel, it's difficult to say for certain who all the main characters are.
Since the title mentions Yuuki Yuuna as a hero, she is surely a main character. In light novels like this, there are often other important characters around the main hero. For example, there could be her best friend who is always by her side, providing emotional support. There might also be a wise elder figure who gives her guidance on how to use her powers. And of course, there could be villains or rivals that she has to face off against, which would also be key characters in the story.
Since I haven't read 'isekai wa smartphone novel', it's hard to say. However, in many isekai novels, the main character is an ordinary person from our world who is suddenly thrust into a strange new world. In this case, it could be someone who is really attached to their smartphone and when they are transported, the smartphone becomes a crucial part of their new life. There might also be characters in the new world that they interact with, like magical creatures or other humans who are intrigued by the smartphone.