Beowulf is strongly driven by his sense of loyalty, but he also has a desire for fame. This desire is less focused on personal notoriety and more on nationalism and bringing pride and strength to his family (the Geats royals). He defeats three monsters: Grendel, Grendel's mother, and the dragon. Beowulf has all the qualities of a good leader: he is kind, generous, and diplomatic as a king, and he keeps the country in relative peace for 50 years.
Hrothgar is king of the Danes and saved Beowulf's father from certain death, thus earning the loyalty of Beowulf. He builds Heorot Hall. His pride for the lavish hall may or may not have contributed to the evil that invades in the form of Grendel. Grendel kills many of his men, eventually leading King Hrothgar to accept Beowulf's help to defeat the beast. Hrothgar is indebted to Beowulf, and they become like father and son.
Grendel is the beast that—annoyed by the noise and partying that goes on in Heorot Hall—terrorizes all who gather at the hall, leading to the hall's desertion for 12 years. Grendel is called many names: the "terror-monger," "hell-serf," and "captain of evil." He is a descendant of the murderous Biblical character, Cain. His arm is ripped off by Beowulf and he is eventually beheaded.
Queen Wealhtheow was married to Hrothgar to maintain peace between his kingdom and that of her parents. She is a gracious hostess who praises Beowulf and asks him to advise her sons. She counsels Hrothgar that he should adopt Beowulf if he wishes, but that his nephew Hrothulf should be named heir to the throne.
The dragon is another evil, fire-breathing creature obsessed with gold. He has taken ownership of a hoard of gold placed in a barrow by the lone survivor of a group of people. The dragon awakens to find a gold goblet missing and wreaks havoc over King Beowulf's land in search of the gold and the person who stole it. The dragon is hunted down and killed by Beowulf—but not before he fatally wounds the hero.
Grendel's mother doesn't actively seek out humans until Grendel is killed. She then takes revenge by killing one of Hrothgar's chief elders and retrieves her son's arm before heading back to her hall under a bloody, boiling swamp. Beowulf dives down to find her and kills her with a magical sword.
Wiglaf is a trusted companion and a cousin of Beowulf. He helps Beowulf defeat the dragon when the rest of Beowulf's men run and hide. In his final moments, Beowulf gives Wiglaf his battle armor. The poem ends before we know Wiglaf's future, but because Beowulf has not proclaimed an heir and Wiglaf perceive the warriors as weak, he predicts wars, slavery, and doom for the Geats.