Ponce da Quirm
Ponce da Quirm (no relation to Leonardo da Quirm) went searching for the Fountain of Youth and spent all his life in the search, prompting Rincewind to ponder whether he'd have been better off living his life instead...
He was encountered in Eric, deep in the Tezuman jungles, captured and due to be sacrificed, still searching for the legendary Fountain of Youth.
He reappeared later in the same book in Hell, still accompanied by the possessed parrot presented to him by Rincewind as a hate-present. Having apparently found the legendary fountain just before his death, he lamented that nobody ever mentions the most important detail about it - "boil it first". (The Discworld Mapp confirms that he died from various water-borne diseases immediately after drinking it.)
Annotations
Based on the real-life Juan Ponce de León (although possibly really used for the genuine comedic propensity of his first name).