Short answers, pulled from the story.
Isildur is a fictional character in J. R. R. Tolkien's Middle-earth, the elder son of Elendil and a descendant of Elros, founder of the island kingdom of Númenor. He became High King of both Arnor and Gondor after the defeat of Sauron at the end of the Second Age, but was killed by orcs at the Gladden Fields shortly after.
Isildur kept the One Ring rather than casting it into the fires of Mount Doom because he was influenced by its power and claimed it as his own, despite the urging of Elrond and Círdan, Gil-galad's lieutenants. The Tolkien scholar Tom Shippey notes that Isildur's own scroll describing the Ring uses the word "precious", signalling he was already becoming addicted to it.
Isildur was killed on the 5th of October in the second year of his reign at the Gladden Fields, ambushed by orcs from the Misty Mountains. He put on the One Ring to escape under invisibility, but the Ring slipped from his finger while he swam the River Anduin, and the Elendilmir gem he wore betrayed his position to orcs on the far bank, who killed him with poisoned arrows.
The One Ring slipped from Isildur's finger while he swam the River Anduin and became lost in the river. It eventually passed to Gollum and then to Bilbo Baggins, as recounted in The Hobbit, providing the central premise for the quest in The Lord of the Rings.
In Peter Jackson's The Lord of the Rings film trilogy, Isildur is played by Harry Sinclair, appearing in flashback sequences. In the streaming series The Lord of the Rings: The Rings of Power, the young Isildur is played by the English actor Maxim Baldry.
In Isildur's youth, the King of Númenor, Ar-Pharazôn, was persuaded by Sauron to cut down Nimloth the White Tree. Isildur entered the royal court in disguise and stole a fruit from the tree before it was felled. He was severely wounded during his escape, but the line of the White Tree was preserved through the stolen fruit, which Isildur later planted in Minas Ithil and, after Sauron destroyed that tree, again in Minas Anor.