(n) A person who likes to be around sad people so that they can feel needed and important. Often, these people will also internalize others' sadness so that they, in turn, can be comforted. They may seem like fragile, weak little birds but a true Leech is just an attention whore who likes the condolences and hugs they get from being sad and fragile.
Grief Leeches are most easily identified by their constantly talking about various friends that they're always "so worried" about. Note that you are always hearing about how she is helping them from the Leech herself and not from her supposed friends.
Person 1 - "God, when Nick and I broke up, Rose came over to 'comfort me' and I ended up having to comfort her! She wouldn't stop crying about my loss. And then, I had to spend the next week hearing everyone else tell me what a great friend she was because, of course, she had told them all about how she was 'so tired' because she had come over to help me in my time of need. What is that!?"
Person 2 - "OMG, Rose is such a Grief Leech!"