Since I'm not acquainted with this light novel, we can only make some educated guesses. The emperor is surely a key character. There could be a young and ambitious person, perhaps a commoner who by some twist of fate ends up on the emperor's lap. This character would then be thrust into the complex world of the imperial court. There might also be antagonists, like jealous nobles or power - hungry ministers who oppose the main character's new - found position close to the emperor.
I'm not sure as I haven't read it. But typically in a light novel, there would be a central character who has a special connection to the 'majesty' in the title. Maybe a servant or a noble who gets involved in the majesty's affairs.
Since I'm not familiar with this particular light novel, it might be a fictional account of a time in a historical - like imperial setting. It could tell the story of a character who finds themselves in an unexpected situation on the emperor's lap. This could lead to all sorts of events, such as being thrust into important political decisions, facing the envy of others in the court, or even having a role in shaping the future of the empire through their new - found closeness to the emperor.
Since I haven't read the novel, I don't know who the main characters are.
The main characters are clearly Nico and Will. Nico is often a complex character with a dark past in many fanfictions, while Will is usually portrayed as more of a healer and a source of light. Their interaction, especially with Nico on Will's lap, is likely to explore their relationship dynamic.
Maybe there is a female character who is either the emperor's love interest or a powerful sorceress who has some connection to the sword. She could be helping or hindering the emperor in his journey with the sword. And there could be an antagonist, perhaps a rival emperor or a dark lord who wants to take the sword for his own evil purposes.
Since it's called 'emperor's domination', the emperor must be a central figure. There might also be his advisors or generals. Maybe there are some characters who oppose his rule, like rebel leaders or ambitious nobles that could also be important characters in the story.
Typically, the main character is the emperor himself. He could be a young and inexperienced one at the start, like in 'The Novice Emperor's Journey' where the emperor has to learn how to rule while facing various threats.
The main character is of course the Magus Emperor himself. He is the central figure driving the story forward with his growth and adventures.
The main characters could include the Star Emperor's closest friends from his youth who now hold important positions in his empire. There may be a wise old sage who guides the Star Emperor in his decisions. Also, an enemy general from a rival star empire who is constantly plotting against the Star Emperor could be a major character. These characters would interact with the Star Emperor, creating a complex web of relationships, power plays, and storylines within the light novel.
I'm not sure specifically as it can vary from one 'emperor of the galaxy light novel' to another. But often there is the emperor himself, who is a central figure with great power.