Living Emotional Crutch

Someone whose presence another character relies on to remain stable.

This person  provides another person with the emotional, psychological, and/or moral support to function. Sometimes to function at all. This could be either someone's first friend or best friend, their love interest, a confidant in the psychological works or a preist.

If something happens to them, this can cause immense anger, insanity or even depression.

Arnold to Helga

Mulder and Scully to each other

Lydia to Lincoln

Connie to Steven