A negative person who tends to ruin other people's good times.

Someone who is over-emotional. One who dwells on their negativity.

Someone or something that could have utility but chooses not to.

This is from the idea that a blanket could be a wanted item, but is no longer wanted when wet.

that one triplet on My Super Sweet Sixteen they keep referring to

A blanket that is wet.

After having sex with a girl, you go into the bathroom and wipe the pussy juice off of your dick with a washcloth. When returning to the bedroom, you throw the washcloth on the girl's face. Thus giving her a wet blanket.

Someone who soaks up cum instead of doing anything fun.