Largely agree with FumbleFingers. An additional thought:
Adjectives can be used as nouns, meaning the idea of that thing, or a person or object that is in some way representative of that thing.
For example, one might write, "How can we know the good?" Meaning, how can we understand this idea of "good", or how can we know what things qualify as "good".
It's a short step from there to characterizing a particular individual as an example of the adjective. "I think that Fred best meets our ideal of the Diligent."
And from there it's a short step to giving someone or something a title reflecting this characterization, "Ethelred the Unready", "America the Beautiful", and so forth, as FumbleFingers discusses.
And sure, in most cases I don't think such a title would be understood to mean that we believe that that example is the only example of the abstraction, or even necessarily the most or the best. Just that it is AN example. Hagar the Horrible is not the only horrible person in the world: even being of Norwegian ancestry myself, I must concede that there have been other horrible people in history. I don't suppose he's even the most horrible. Just that he is horrible.