This version of Mary Sue is often put into action by authors who think that a character can be made likable by writing them as The Woobie. And then cranking it Up to Eleven. Her life is packed with Deus Angst Machina either in her backstory or in the actual story she is in. She collects tragic events the way normal people collect baseball cards. And she is written with the sole intention of making you feel sorry for her. Like most Mary Sue subtypes, they can be male or female, but generally tends towards the latter since women are perceived to be more sensitive and vulnerable.
Please note that you can create an angsty character who isn't a Mary Sue. Even Wangst can have its place in good fanfiction, if done right. An unhappy history and a gloomy personality aren't the only things that make a Sympathetic Sue. The problem with this brand of Sue starts when the writer confuses a horribly traumatic backstory for a genuine character flaw, and only gets worse.
A good angsty character's emotional pain is never stylish, and does not necessarily involve weeping and wailing. People deal with pain in many different ways, usually by becoming trapped in one of the Five Stages of Grief. If they do resort to self-harm such as cutting themselves, it's not glamourised. And like any normal person who is grieving, there will be a justifiable reason for it, because when pain is great, talking about it is therapeutic and keeping mental anguish bottled up inside can be very unhealthy and result in severe problems later on. And the grieving will be because the character is actually in pain, not because they are trying to get sympathy from the audience.
However, justified angst can also be Wangst if the character grieving about it is out-of-proportion, and whether the grieving is out-of-proportion depends on the situation.
Their feelings of guilt will make sense, usually because of psychological scarring. Perhaps, just perhaps though, they were responsible for something that happened to someone else and are dead on with their remorse. They might have tried to do something to relieve the pain in the past, even if it failed. And canon characters do other things as well as comfort the angsty character. Even after the angst is gone, there's still a story to be told.
Common Sympathetic Sue backstories involve:
- Abusive Parents
- All of the Other Reindeer
- Child by Rape
- Domestic Abuse
- Doomed Hometown
- Last of His Kind
- Parental Abandonment
- Rape as Backstory
- Survivor's Guilt
Keep in mind that these tragic background elements do not, by themselves, define the character as a Sympathetic Sue. Any of these traumas is a good reason for any character — or any living person, for that matter — to suffer some form of depression. And your character can have more than one of them without being a Sue, although the more you have, the harder it will be to write them realistically. But Sympathetic Sue will have suffered hugely — sometimes from all of them at once. Not that this doesn't often happen in real life, but it's not cool.
Apart from all that angst, there are all the regular Mary Sue things: Authorial intrusion, plot favoritism, unnatural magnetism, and such often also with a lack of perceivable flaws and a strong tie to the author. It's just that there's something for her to be overly and usually unrealistically depressed about. Perhaps the most important factor in a Sympathetic Sue is that, no matter what she's going through or how emotionally or physically damaged she may be, these damages never make her ugly; if she is unkempt she will be an Unkempt Beauty, any terrible ordeals she goes through will usually result in an Adrenaline Make Over and if she is ever physically scarred it will be a cool looking, Bond villain type scar that will function more as a decoration (and a reminder of how beautifully and poetically tortured she is) than an actual injury. Sympathetic Sue always suffers beautifully (possibly a variation or Beauty Is Never Tarnished.)
But Sympathetic Sue's unhappy past doesn't really match up with how much she angsts over it. No matter how many traumatic events her history involves, the angst is still not portrayed realistically. Often, it doesn't even have any real permanent implications; it's just a reason to gain the attention of a true love who will spend most of the story trying to make her feel better about it. Essentially, it's not an important part of the story, or the character. It's just there to make people feel sorry for her.
She'll talk about her pain constantly and as soon as possible. In Real Life, some people are very quiet about past trauma, and simply don't share with anyone if the trust isn't there, or if they're not ready to talk. On the other hand, other people are very open about their emotional pain because talking about it is therapeutic. A problem shared is a problem halved, people! But Sue's sharing will simply double it.
Sue often blatantly ignores all the positives in her life. Self-blame is often irrational, but Sympathetic Sue takes Survivor's Guilt to its most extreme state, blaming herself for her parents dying in a plane crash, and not just feeling mild guilt either, but wallowing in it deeply and extensively for years on end to get attention. The other characters will never get tired of trying to cheer her up, even if it's totally out of character for their personality. If they're not near her, they'll probably be discussing how sorry they feel for her.
She'll never attempt to relieve the emotional pain herself — other characters do all the legwork for her. The implication is, if she moves out of her pain too quickly — or at all — it was never that big of a deal in the first place. Any character who doesn't try to help gets chewed out or portrayed as a jerk. And most of all, the story comes to an end as soon as the angst is gone.
In a lot of cases, the character is not a Sympathetic Sue in the original work, but appears as one in fanfiction. Authors, in attempt to generate more drama or sympathy for their favorite character, sometimes exaggerate said character's past trauma or write a new scenario in which he/she is heaped with Wangst.
Pairs well with Jerk Sue, since angst can fuel anger and acting out — but, you know, sympathetically. Generally shows up in Hurt Comfort Fics, and is a common fate for an in-series Woobie or Draco in Leather Pants. May also be a Damsel Scrappy.
Polar opposite of Iron Woobie, a hard knock character who fights on to improve their lot and refuses to give in to their grief.
No examples please, this just explains the term.