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.
The main character, Hachiman, is surely important. His thoughts and actions usually drive the story forward.
Hachiman Hikigaya is a main character. He is a cynical and introverted high school student with a rather pessimistic view of the world and human relationships.
One of the main themes could be personal growth. Characters might continue to develop and face new challenges in this volume.
Yukino Yukinoshita is also a main character. She is a beautiful and intelligent girl but has her own set of problems and insecurities. And there is also Yui Yuigahama, who is more of an outgoing and friendly character, often acting as a bridge between Hachiman and Yukino.
Iroha is one of the main characters. She has her own unique personality and plays an important role in the story.
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.
One of the main characters is Hachiman Hikigaya. He is a cynical and introverted high school student.
One key theme could be self - discovery. The characters might continue to explore their true selves as the story progresses in this volume.
Since I haven't read '86 volume 2 light novel', I can't give you the exact main characters. However, in many light novel series, the main characters are often the ones who drive the story forward. They are likely to be the ones with the most complex backstories, goals, and relationships. Maybe in this volume, some new characters are introduced who become main characters or the existing ones from volume 1 have their roles expanded.
Hachiman is a key character. He is a cynical and introverted boy who has a unique way of looking at the world. Yukino is also important. She is a smart and reserved girl. And Yui, who is a cheerful and friendly girl, is another key character.