Sticker

Someone (usually a male) who overly decorates themselves to look all adorable for someone they’re attached to and heavily obsessed with

A Sticker will also do almost anything it takes for that person they’re attached to; but they aren’t technically Yanderes. They are overall harmless. They’re just affectionate.
Stickers also tend to act younger than their true age.

Someone’s extremely attached to another, loves them very much, decorates themselves, and is incredibly immature? Sounds like a Sticker.