Melanie Martinez - Teddybear: https://www.youtube.com/watch?v=NJ6jv_lPBN8

Either a literal teddybear or someone with that nickname. A helps B gain self confidence. B changes. Gets possessive? A gets possessive? Roles get reserved?

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"Teddybear" has problems. Yoonbum style? Nobuta style?
A helps T. 
A feels jealousy and starts acting bossy?
They get together.
T ends up possessive and obsessed.