Arathorn II, son of Arador, was the father of the King of the Reunited Kingdom, Aragorn II Elessar. But that is not the only thing he is known for.
He was the fifteenth Chieftain of the Dunedain, and ruled the shortest. When he had become the Chieftain in 2930 (His father was killed by hill-trolls) he only had enough time to produce his child and heir, Aragorn II, before he was shot through the eye while hunting the orcs as his great-great-grandfather, Arassuil, had first done when the came into Eriador.
Before his death, he married Gilraen the Fair, who was only half his age. The marriage was opposed by Gilraen's father, Dirhael, who was a descendant of Aranarth himself.
'"Moreover," he said, "Arathorn is a stern man of full age, and will be chieftain sooner than men looked for; yet my heart forbodes that he will be shortlived."
Arathorn died at sixty years old, and his son, Aragorn II, became the chieftain at age two.