Holling Hoodhood is both. He is not both at the same time. That would be weird. Holling is a dynamic character, which means that he changes and develops throughout the story.
I would say that when the story begins, Holling is quite pessimistic. I would even say that his feelings are justified too. His dad is a crazy workaholic who can't find the time to show interest in his kids. Holling is not Catholic or Jewish, which makes him the only kid with no place to go on Wednesday afternoons. He feels like a bit of an outcast. To add to those feelings, he must attend a special study hall with Mrs. Baker on those Wednesday afternoons. Mrs. Baker is not nice at first. She questions Holling about everything and gives him mundane jobs like cleaning chalk erasers. Holling doesn't have a lot to be happy about.
As the book progresses though, Holling begins to become much more optimistic. Mrs. Baker actually begins teaching Holling. They work through various pieces of Shakespeare, and Holling learns to like it. In fact, he signs up to be in a Shakespeare play. Holling takes up running, and Mrs. Baker just happens to be a former Olympian. She begins to coach him. Holling gets a few close friends, and even discovers that the girl he likes reciprocates those feelings. Holling's dad is still a jerk, but Holling matures enough to not let it affect him the way that it did before.
If you want a book about a pessimistic character through and through, read "The Catcher in the Rye."