From the Super Mario Wiki
Neptune, Lord of the Deep, better known as King Neptune is a merman, and the ruler of the Aqua Land. He is the king of the Mermushrooms, despite not actually being a Mermushroom himself. King Neptune generally has a very good disposition, displaying a very calm and pleasant attitude unless he is pushed too far. Despite his peaceful nature, he is not afraid to fight against anyone who stands in his way.
King Neptune's reign over Aqua Land was brought to a halt when Barra-Koopa and his Bloober minions invaded Aqua Land and enslaved its entire Mermushroom population. King Neptune tried to reason with Barra-Koopa without threatening him, but Barra-Koopa disregarded his advice, and ordered for him to be locked up in his own dungeon.
Locked away from the rest of society, King Neptune could do little about his situation until help arrived. Led by a female Mermushroom, Mario, Luigi, Princess Toadstool and Toad broke into the dungeon and rescued King Neptune. Now free, King Neptune agreed to help them rid Aqua Land of Barra-Koopa, with the help of his fishy friends.
King Neptune led the charge by riding atop a Whale, which he used to smash a hole in wall leading into his own throne room. Mario's group then entered the throne room on four of King Neptune's seahorses, and used his swordfish to fight against Barra-Koopa and his Bloober trio. Eventually, the Koopa Pack was defeated, and King Neptune thanked Mario's group for their help in saving Aqua Land.