The Traveler is extremely important. They are the main protagonist, on a quest to find their sibling. Then there's Paimon, the Traveler's sidekick, who provides comic relief and also helps in explaining things to the player. Zhongli is also crucial. As the former Geo Archon, his knowledge of Teyvat's history and his influence on Liyue's development are significant parts of the story.
The free story - related characters in Genshin Impact have multiple advantages. Firstly, they are free, which is a big plus for new players who don't want to spend money right away. Characters like Amber are good for learning basic combat mechanics as her ranged attacks are straightforward. Lisa has electro abilities that can stun enemies, which is useful in various battles. Kaeya can freeze enemies with his cryo powers, providing crowd control. They also help in understanding the elemental reactions in the game as they each represent different elements. Moreover, they are part of the story, so they give a sense of connection to the game's world.
As of now, the Genshin manga is not finished. The story is still evolving and there's more to come for fans to enjoy. The creators are taking their time to develop a rich and engaging narrative.
There's no definite answer yet. It might continue if there's enough demand and the creative team has more story to tell. But it could also stop if they feel they've completed the story arc.