Healfdane, son of Beo, had three sons: Hergar (or Heorogar), King of the Danes, Hrothgar, who became king after the death of Heorogar, and Halga, the youngest. Healfdane also had one daugher, Yrs. See lines 57-63 of the Prologue:
The great Healfdane, a fierce fighter
Who led the Danes to the end of his long
Life and left them four children,
Three princes to guide them in battle, Hergar
And Hrothgar and Halga the Good, and one daughter,
Yrs, who was given to Onela, king
Of the Swedes...
(Burton Raffel trans.)
Hrothgar is the only one of the sons who figures prominently in the poem. Hrothgar is an aging king, beloved by his people, builder of the great hall Heorot. When the monster Grendel begins his deadly attacks on the hall, Hrothgar welcomes Beowulf and seeks his protection. Unlike Beowulf, who is able to accomplish great feats through arms, Hrothgar earns his people's love through his compassion, his generousity, and his wisdom.