Another theme could be redemption. The assassin, having done bad things in the past, might be seeking to atone for his sins through his new life as a farmer. He could be trying to make up for his previous actions by living a more virtuous life, growing crops and helping the local community. This theme of transformation from a 'bad' person to a 'good' one is quite common in such light novels.
One main theme is growth. The spirit farmer has to grow not only his crops but also his skills and knowledge in dealing with spirits. This could involve learning new spells or techniques to make the spirits more productive.
Another theme is power and how to use it. In these novels, the assassin usually has some skills from their past life. They need to decide how to use those skills in the new world, whether for good or bad. For example, in 'The World's Finest Assassin Gets Reincarnated in Another World as an Aristocrat', the main character has to balance his assassin skills with his new aristocratic identity. It's a complex exploration of power dynamics.
One popular 'assassin farmer' light novel could be 'The Assassin Who Became a Farmer'. It tells an interesting story of an assassin who gives up his old life and starts anew as a farmer, facing various challenges in the rural 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.
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.
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?
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.
Love might be a theme as well. In many stories, when one person is saving another, especially in a scenario like this, romantic feelings can develop. There could also be themes related to the female assassin's past, like her reasons for becoming an assassin in the first place, which could be explored as part of the story, such as vengeance or being forced into the role.
Typically, the main character is the assassin themselves. They are often highly skilled in combat, stealth, and have a sharp mind. For example, in many Hidden Assassin novels, the assassin might be a lone wolf type, like in 'The Silent Assassin' where the main character, Kai, works alone and has his own code of honor.
One main theme is hard work. Chinese farmers work long hours in the fields to ensure a good harvest. Another theme is their connection to the land. They respect and depend on the land for their livelihood.