It's a side story related to the main 'Solo Leveling' series. It likely delves deeper into the world, perhaps exploring the backstories of some characters, or events that are not fully covered in the main story.
Since it's a side story, it could introduce new characters while also featuring some familiar ones from the main series. However, without more specific information, it's hard to say exactly who the main characters are. But it's likely that some of the key figures from the original 'Solo Leveling' will be involved to some extent, whether directly or through their influence on the events in the side story.
Solo Leveling is definitely a manhwa. Manhwa have their own style and fan base. Solo Leveling stands out among them with its fantasy elements and exciting adventures that have attracted readers worldwide.