I'm not sure specifically as I haven't read it. But generally, a light novel often contains elements like unique characters, an interesting plot, and a fictional world. It might be about the relationships between a beauty (bijo), a sage (kenja), and a demon (majin), perhaps involving their adventures, conflicts, or a quest of some sort.
Well, 'bijo to kenja to majin no ken light novel' could potentially tell a story where the characters - the beauty, the sage, and the demon - are in a struggle for power or knowledge. Maybe the beauty has some sort of special ability that the sage wants to study and the demon wants to use for evil. Or it could be a love triangle where the sage and the demon both have feelings for the beauty, which complicates their relationship further. However, without reading it, this is all just speculation.
Since I haven't had the chance to read 'bijo to kenja to majin no ken light novel', it's hard to say exactly what it's about. But light novels usually have a central theme or goal for the characters. In this case, the three main characters - the girl (bijo), the wise man (kenja), and the demon (majin) - might be on a journey to save their world from an unknown threat. The sage could use his wisdom, the beauty might have some hidden strength, and the demon could have some power that is both a curse and a blessing in this fight. Or it could be a story more focused on their personal growth and how they interact with each other in a complex social or magical setting.