This is a manipulation and abuse tactic. You do something she doesn't like she threatens suicide you relent she feels in control. She then uses it to control everything and gain all of your attention. This is a common tactic. Yes she may be depressed but abusive people can also be depressed.

-you are a teen and not a lisenced therapist. If you try to help her and fuck it up you will understand that you are way in over your head.

-This is a problem for the adults in her life to deal with. You need to tell her parents every time immediately when she says something threatening to herself or others.

-this is not your fault or your problem. This is not your responsibility.

-relationships and friendships are for positive social interaction for your enjoyment. She is not fun get a better friend.

-threatening suicide or self harm is different that her just needing someone to confide in or a shoulder to cry on once in a while. If she is saying things like if you ever leave I will kill myself that is her being abusive and mentally unstable she needs help at that point from adults.

-there are cheery sweet girls that want to bake you cookies and play with your dog. Dont waste your time with this. Dont let your adult life get messed up by an abusive relationship in your teens.