A constitution is a statement of the basic principles and laws of a nation, state, or group, such as the U.S. Constitution. Another very common meaning of constitution is the physical makeup of a person. So think of a constitution as how a body (yours, the government's) is set up. If you have a strong constitution, it means you don't get sick very often.
I think the focus of the story is on Beowulf as a hero rather than a king because it is his heroic deeds that lead to him being offered the throne. Also to be king one must possess some type of heroic qualities in order to rule a land and its people.
Every character in a poem is unique in their own way. In the poem Beowulf, Beowulf endures many challenges that make his character stand out from the rest. Beowulf is a major character with a dynamic, yet flat personality. Beowulf is considered the major character because he has the most influence on the plot. He receives the call to Heorot while at home in Geatland when he hears that Grendel is attacking Heorot.