Another theme might be power and its consequences. As an assassin, he has a certain kind of power. The novel could explore how this power affects him and those around him. For instance, does it isolate him or attract unwanted attention?
The world - building in this light novel also plays an important role. It could be a fantasy world with unique magic systems or a more historical - like setting. The settings would affect the assassin's actions and the overall atmosphere of the story. If it's a fantasy world, there might be magical creatures and spells that the assassin has to deal with.
The main character is, of course, the world's greatest assassin who gets reincarnated. But without having read the entire novel in detail, it's hard to say exactly who else might be important. There could be a mentor figure in the new world who helps him adapt.
One of the main themes is redemption. The assassin has to come to terms with his past actions. Another is power and how it can be used or misused.
One reason is its unique concept. The idea of the world's greatest assassin being reincarnated into a different world brings a fresh take on the isekai genre. It allows for interesting character development as the assassin has to adapt to a new environment with different rules and powers.
One of the main themes is probably pride itself. The characters might struggle with their own pride, whether it's a noble's pride or a sense of self - worth. Another could be about power and how characters use it in the context of the story's world.
Well, power and its corrupting influence can be a theme. As the assassin gains new abilities in the reborn state, they may face the temptation to use it for wrong reasons. Also, loyalty is a theme, whether it's loyalty to an old cause or to new allies they make along the way.
There could be side characters as well, like a mentor figure who taught the assassin their skills, or a love interest who adds a different dimension to the assassin's character, making the story more complex and engaging.
I'm not entirely sure without reading it, but there's likely a main assassin character. Maybe there's also a mentor figure who trained the assassin.
There are likely to be other important characters such as the people who train the assassin, his targets, and perhaps some allies or friends he makes along the way. However, without reading the entire light novel in detail, it's hard to name every single main character.
The central character is the assassin. Alongside him, there could be some powerful villains who oppose him. And there might be some side characters such as a weapon smith who provides him with special weapons or a informant who gives him crucial information about his targets.