I'm not entirely sure without reading it, but it's possible that no new major characters are introduced and the focus remains on Kazuma, Aqua, Megumin and Darkness.
In Konosuba vol 8, while there may not be completely new major characters, there could be some minor characters that play important roles in the specific plot of this volume. However, the core group of Kazuma, Aqua, Megumin and Darkness will no doubt be the central characters. Kazuma is the leader of the group, trying to navigate their crazy adventures. Aqua is the goddess who adds a lot of comical elements. Megumin's powerful explosion magic always creates a big impact, and Darkness has her own unique personality quirks that contribute to the overall story.
Kazuma is surely one of the important characters as he is the main protagonist in the series.
Ainz Ooal Gown remains the most important character. His decisions and actions drive the plot forward. He is constantly strategizing about how to expand his influence in the new world.
I'm not entirely sure about brand - new characters specifically in Vol 8, but existing important characters like Ainz Ooal Gown continue to be central. His actions and decisions drive the plot forward.
One important character introduced could be some of the new leaders of the local factions in the New World. They play a role in the political and military aspects that Ainz and his forces interact with. For example, there might be a new king or a powerful general.
The important new character in Durarara Vol 5 Light Novel is Aoba Kuronuma. He's a complex addition to the cast. Aoba has an air of mystery around him. He quickly becomes enmeshed in the power struggles and the web of relationships in Ikebukuro. He seems to have his own agenda, which isn't fully clear at first. As the story progresses, we see how his actions and his personality start to shape the events. He also has an impact on the way the other characters view their own situations and the state of the city as a whole. His presence really shakes things up in the already chaotic world of Durarara.
One important character introduced could be new allies that Rimuru meets. They might have special abilities or knowledge that help Rimuru in his adventures. For example, there could be a powerful mage or a skilled warrior.
The main characters in the KonoSuba series, which likely are also the main characters in vol 16, are Kazuma, Aqua, Megumin, and Darkness. Kazuma is the cynical and somewhat perverted protagonist. Aqua is a useless goddess. Megumin loves her explosion magic, and Darkness is a masochistic crusader.
In Konosuba Vol 6, Kazuma is the leader - like figure of the group, trying to keep everyone in line. Aqua provides a lot of the comical relief with her lack of practical skills. Megumin's explosive magic is a key part of their combat strategy. And Darkness adds an interesting dynamic with her masochistic tendencies. Their different personalities clash and combine to drive the story forward in this volume.
Since the series often focuses on military aspects, there might be new comrades or rivals within the military for Tanya. They could be soldiers with unique skills or personalities that either help or hinder Tanya's progress. Also, there could be some civilian characters who get involved in the military - political situation, like diplomats or spies, that start to play a role in Tanya's story in this volume.
There might be a new mentor - like figure who has a unique perspective on magic. Maybe there's also a new rival, someone who challenges the main character in his pursuit of knowledge or power. These new characters would add more depth and variety to the story.