The Idealist
A character who genuinely believes that their world is A World Half Full; that Humans Are Good, or at least that Rousseau Was Right and a person who will tell you that if you think it's wrong to hope that you're wrong every time. They will take ideals that others have for the future and will do everything they can to take them to fruition, sometimes going too far.
If you begin to deconstruct their idea, they will immediately begin to reconstruct it. This character will take their (perceived) Crapsack World, and keep moving forward.
The way that an idealist can do this even in the face of adversity and certain death is by focusing primarily on non-material things. Of particular note are the famous/infamous trio of faith, hope and love. Though having any religious grounds for their worldview can usually explain an idealistic perspective, and can be played both positively and negatively.
In fiction, (and perhaps as an instance of Truth in Fiction) idealists are often seen in the role of the main character- usually The Hero- for their tendency to act from internal motivation. However, they can take other forms if idealism is being heavily criticized by more cynical writers, or played as the villain. A big draw of putting them in a main position being: "No revenge plot necessary!" They motivate themselves.
This character is likely to show up in any work. However, they can exist in a Sugar Bowl, where they are always right, or in a Crapsack World, where they exist solely to be proven wrong (and often brutally killed for drama).
The polar opposite of The Cynic. See also Idealism Tropes, and the Sliding Scale of Idealism Versus Cynicism.
Sub-Tropes:
This is a Super-Trope, so examples should go to the relevant sub-pages if possible.