Hell yeah that's a really interesting one! The format I'll do for this is pasting my own definitions of the class and aspect and then getting into them.

Knight: Someone who serves and protects their aspect, and serves and protects others and themselves through their aspect. This is accomplished by either using it as a weapon or making more of it, as there usually not very much of their aspect in their party. 

Light: Knowledge, particularly narrative knowledge and narrative importance (spotlight), fortune, luck, literal light, foresight, literal sight. Often associated with emotional instability.

So! I think Knight of Light would fall equally into the serving and protecting their party through their aspect, and serving and protecting the light in their session. Most sburb sessions start out with a lack of light anyway, as no one goes in already understanding the game. Their moon actually is significant because it would be the main way they gain their insight, like Rose did through the Horrorterrors on Derse. They would either be awake before entering the game or wake up very soon after entering their medium, or wake up right after the first person in their party enters their medium, and use the knowledge they gain from the clouds to guide themselves or their friends along. They'd be very similar to Rose, but their actual ability to gain knowledge about EVERYTHING would be debuffed in exchange for using their powers for more combat abilities, as well as a duty to protect their sessions narrative importance and luck.

Their actual tangible powers would be random visions or premonitions that help them figure out how to play the game, and also to help them fight. They would know what moves would work, being able to predict their enemies attacks, and being intuitive, knowing where they need to go. They would be able to pull off some absolutely ridiculous Nagito Komaeda type bullshit and use their luck in a fight. Taking huge gambles for huge rewards in battle.

Similar to Dave, it'd be their duty to protect their timeline from becoming doomed. They'd be a huge supporting character and always lead their party members towards doing the things that end up being important in their story. Classpects are all about how characters interact with their story, which is why so many classes and aspects have to do with the narrative. A lot of light players are basically walking plot devices that an author can use in different ways depending on their class.

Hope this is good!