Koopley is Koops' father that went to challenge Hooktail ten years prior to the events in Paper Mario: The Thousand-Year Door. Koopley managed to defeat Hooktail, but as he was about to finish her off, Hooktail tricked him into being merciful and devoured him. Fortunately, Koopley survived by curling into his shell. During his time in Hooktail's stomach, he found the Crystal Star she was guarding and took it as a souvenir. When Mario and his friends defeated Hooktail, she regurgitated Koopley, who became reunited with his son and gave the Crystal Star to Mario. He likes eating chicken and can be really aggressive when taken away.
"Well, I came here a while back to deal with Hooktail, and all was well and good... But just before my finishing blow, he played a trick on me and gobbled me down whole!”
-Koopley, upon being rescued from Hooktail's stomach