Briefer

That friend who you have to brief your other friends on before you introduce them because the friend is a bit weird and/or embarrassing and you don't want your friends to judge you for them.

So my friend Stacy is coming tonight, she's a bit of a briefer. She makes really shit jokes and gets wasted on 2 drinks.

briefer

A more brief, concise, shorter, or simpler, word or phrase.

Thought the word "dysfunctional" is more common and has 1 syllable less, the word "inoperative" is briefer. The first has 13 letters and 4 syllables, the second has 11 letters and 5 syllables.