This person, let's call her Alicenote , provides a troubled character, let's call him Bob, with the emotional, psychological, and/or moral support to function. Sometimes to function at all. She might be his first good friend, a Love Interest, a psychiatrist/priest Confidant, or a full-blown Caretaker to Bob, serving as a Morality Chain or Kid with the Leash to rein in Bob's darker tendencies, deliver pep talks and administer a Cooldown Hug.
Summed up, while she's alive and near him, Bob is a better, happier person.
This is why she will usually get kidnapped or killed. Bob's reaction will be one of two extremes: a psychotic Unstoppable Rage on par with The Hulk in the throes of withdrawal from Gamma-Cocaine, or comatose gibbering; see also Went Crazy When They Left. The middle road is a Tranquil Fury of deadly determination.
There are some dark uses of this trope. If Bob is psychotically possessive, he will go to any lengths to protect Alice from perceived threats, culminating in destroying everyone she loves so she becomes trapped in an All Take and No Give relationship. This is especially tragic if Alice genuinely wants to help him, but he's already sliding too far to help. When she inevitably leaves him/is rescued, Bob's mental state takes a nosedive.
Sometimes, Alice will be able to introduce Bob to other friends, lessening his dependence on her. Very rarely, she will be able to cure him of his dependence entirely, or give him the necessary tools and resolve to survive when she's gone.
This trope has some basis in Real Life. People like Bob might be diagnosed with dependent personality disorder or, if they're cripplingly shy around most others, avoidant personality disorder.
Compare The Heart, when the character is this to the others in their group.
