Although not related by blood, Beowulf the Geat shares a name with a beloved Danish king of old. Hrothgar knew Beowulf's father, and so the two had a respectful allegiance to each other. Hrothgar had interceded on the Geats behalf years before and prevented a war, so both Beowulf and Hrothgar saw Beowulf's coming as a valorous and honorable deed.
Even though Unferth questioned Beowulf's abilities, Beowulf's account of his own victories over sea monsters was sufficient for Hrothgar to see the heroic ideal working itself out in Beowulf and brought him in as a friend and son of the Danes.
Beowulf is one of the oldest surviving epic poems in the Old English language. The manuscript was produced between 975 and 1025, but we do not know when the actual poem was first composed. We also do not know who the original poet was; that mystery person is referred to as the “Beowulf poet.”
In this epic poem, the great hero Beowulf comes to the aid of the tormented Danes. He slays the monster Grendel and then Grendel's monster mother, and eventually kills a dragon. He dies in the final battle with the dragon, but successfully rids the land of its plague of monsters.
One of the requirements for “heroes” in ancient English literature is an inherent imperative duty to help those who are helpless and in danger. Beowulf fulfills this role. Beowulf has no obligation legally to help the Danes. He actually serves a different king and is a hero of the Geats. He is not part of the Scandinavian tribe being attacked by Grendel, a monster descended from Cain. Thus, Beowulf asks his king if he can go to the aid of the Danes and King Hrothgar, and his king allows it. Beowulf leaves his land to go assist people he has no obligation to, and that action makes him a hero. So, even though Beowulf does not know Hrothgar well, he decides to come to his aid and help rid the great hall Heorot of the monster Grendel.