Q1 - It is hard to define which one is the main character in Of Mice and Men, as George and Lennie have protagonism in the story. But George comes first, as he represents the voice of reason of the duo, as the story unfolds.
This story takes place in California during the Great Depression (1930´s), and the two characters above are men without jobs or perspectives, looking for an opportunity to earn money and buy their own ranch.
George´s relationship with Lennie is different as they are close friends, trying to take care of each other. Also, they are both migrants, with all the implications this would bring at the time.
In the ranch where they find work, they meet other characters: their boss, the son of the boss, his wife, and other ranch hands.
These relations are shown in the book illustrating how unfair and unequal was this period, where racism and sexism and other prejudices were the regular concepts in society.
There is Candy, the aging one-handed ranchman who fears being useless and therefore fired. There is Crooks, the black man who is regularly isolated and threatened by other ranch men. There is Curley´s wife, whose dreams were crushed by marriage, and whose neck was broken by Lennie...
These examples remind that this is a drama with no happy-ending, and the realistic, sometimes offensive, way it was written has regarded it with criticism. But the development of the story has great merit in showing how hard those times were.
Q2 - This book is written in third-person, with an omniscient objective narrator. This means that the story is not told directly by one of the characters, despite having direct dialogues and statements from them, as in this excerpt:
"George's voice became deeper. He repeated his words rhythmically as though he had said them many times before. 'Guys like us, that work on ranches, are the loneliest guys in the world. They got no family. They don't belong no place. They come to a ranch an' work up a stake, and the first thing you know they're poundin' their tail on some other ranch. They ain't got nothing to look ahead to.”