There may also be other supporting characters that contribute to the story in volume 12. However, without reading it, it's hard to say exactly who else might be prominent. But the three main ones - Hachiman, Yukino and Yui - are likely to be at the heart of the plot, driving the story forward with their actions, decisions and interactions.
Yes, often the anime covers of 'Oregairu' are based on the light novel. They are designed to give viewers a sense of the story, characters, and overall mood that is present in the light novel. The cover art might feature the main characters in a pose or situation that is significant to the plot, which helps to attract fans of the light novel to the anime adaptation.
It's about the story of the characters in a rather complex and interesting way. It involves their relationships, personal growth, and the exploration of youth and social interactions.
It depends on personal preference. If you like visual and auditory experiences, the anime might be better for you. The anime has great voice acting and animation that can bring the story to life. For example, the expressions of the characters and the background music can enhance the emotional impact. However, if you enjoy in - depth reading and exploring every detail of the story, the light novel is the way to go. It can offer more complex character thoughts and additional plotlines.
Overall, the OreGairu manga follows the light novel pretty well. There might be some minor differences in the way certain scenes are depicted or the emphasis on certain details, but the core story remains intact.
I'm not entirely sure as I haven't read it yet, but if it's in line with the previous 'oregairu' works, it might feature Hachiman Hikigaya as one of the main characters.