bestie
warm, affectionate
the one-line answer
Best friend. Used both sincerely and as a term of address for any friendly person.
Bestie is a cute form of best friend. You call your actual closest friends besties. You also call strangers bestie when you want to be gently corrective or affectionate. Context determines sincerity.
mainstream
Your coworker might use this word.
01 / in the wild
hear it move.
advice
bestie, you cannot wear that to the interview.
Friend, I am giving you honest fashion advice because I care about you.