REE

It depends on the context but can mean frustration or something sad or angry or scared it can express many different types of emotions.

Her: I lost my airpods today :(
me: reee I am so sorry :c
or
Your GF: I cheated on you....
You: REEEEEEEEE!!!!!! HOW DARE U!!!!!!! >:C
or even
GF: I bought you a brand new Playstation for your birthday!! Happy Birthday!!! <3
You: REEEEEEEE I LOVE U SO MUCH <3 :)
or even
Damn this box is heavy REEEEE

or
*steps on a lego* REEEEEEE OW THAT HURTS!!! >;c
last example
BOO!!
REEEEE!!!! Don't scare me like that D;
HAHAHAH :D