How do you spell Wendy in Japanese?

If you mean it just as a name or title it would be ウィニー /wi nii/. If you mean Winnie the pooh it would be 小熊のプーさん /ko gu ma no puu san/ [lit: Pooh the cub] or クマのプーさん /ku ma no puu san/ [lit: Pooh the bear].