Captain Syrup is a high-ranking female pirate who is in charge of the Black Sugar Pirates and has an infamous reputation - she serves as one of Wario's recurring villains and could be considered a nemesis of sorts as she has been the main villain of multiple Wario games.
Like many Wario characters her name is a pun on food - this is a trend a few Wario villains have (such as Terrormisu and Count Cannoli).
Wario Land
She didn't appear in the end, but was the main antagonist and stole the giant Peach statue from Peach herself. Being Greedy, Wario came to Syrup's base, on Kitchen Island, to sell the statue for ransom so he could get a castle. Seeing him as a big threat, Syrup ordered her henchmen to dispose of Wario. After getting to her, Wario fought her and a genie. She was defeated, and blew up her hideout in rage.
Land 2
She stole Wario's Money and ran away with it. Wario fought her many minions. Syrup was so fed up with Wario, she decided to fight him herself in the final battle, as she chucked bombs at Wario. To defeat Her, Wario must chuck the bomb back at Syrup. In an alternate story line, Syrup takes over Wario's Castle when he is asleep.
Shake It
Not appearing for over a decade, Syrup returns in Shake it as an ally of Wario. She owns the Pirate Shop, where she sells Wario maps and heart container. In the end, She betrays Wario and steals the bottomless Coinsack right under his nose.