Before you start writing the most tragic back story ever for your character, I’d like to point out that not all good characters need angst. Its best if used sparingly and not the main focus of the plot. Also, just because someone’s OC has an angsty past, does not immediately make said character a Mary-Sue. If they have angst for all the wrong reasons, then they are a Mary-Sue.
If you are writing certain types of trauma (rape, child abuse, ect.) try to include a trigger warning, unless it is a fanfiction of something that possesses the same exact type of trauma.
Part one: Writing Angst in General
First decide why you want to give this character angst in the first place. If you don’t what your character to be seen as a Mary-Sue, than you must have a good reason for their suffering. A good reason for angst might be to show why your character is the way they are today. Some bad reasons for trauma would be to make people feel sorry for your character, or to make them look more badass for surviving it. Even worse when the goal of the trauma is to have them cuddle with another character.
Choose a type of trauma for your character. Make sure the type you choose works with the cannon, and that you can properly write it (if you have very little knowledge of sex, you won’t be able to write a good rape story). Here is a list of some traumas you can use.
Relative was murdered or committed suicide (they don’t have to have seen it happen)
War related trauma
Loss of a child
Divorce of parents (only in young children
Actually do some research. This is not the only resource you should use for writing abuse. Books are you best bet since they had to be accurate to get into print. If you can’t get books look for sites with .org or .gov.
If no characters have tragic pasts, don't add one in. The character will fit in horribly, and you won't be seen as a good writer if the story in't that angsty. Just something to keep in mind.
Don’t let it become wangst. Whiny angst, or wangst for short is the name of angst where the character never stops complaining about their past, are constantly crying, and saying things like, “You don’t understand me. Nobody understands.” Often the things wangsted about are small, petty issues (like a boy who is 18 and has had no trauma thus far crying about his parents getting a divorce). Readers will not feel any sympathy for a wangsty character, and only think, “Why haven’t they killed themselves yet.”
Have characters eventually get sick of their angst. If you wanna make your OC go on and on about their angst, make other characters constantly be telling them to snap out of it without trying to comfort them. Even the most sympathetic character has a limit. A good example of this would be:
Normal character: Come on, you need to snap out of it. You’ve been like this for days, and there’s a war out there.
Angsty character: But my family just died.
Normal character: Who even cares? A lot more people are gonna die if you don’t snap out of it.
Part two: Writing Abuse
Give the abuser a good reason to hurt the victim. People don’t abuse others for the hell of it; they need a good reason for it. Maybe a man abuses his son because his wife died in child birth of that son.
Abuse should start small and get bigger over time. If the abuser was full on abusive before the victim got really into it, they would’ve left at the beginning. It should get worse as time goes on, and the victim has more invested in the relationship that they’re afraid to lose. The victim may have nothing besides the abuser, and not want to lose the only real thing they have to hold on to.
Victim may think abuse is normal. This is especially true in children, who may see this as a normal punishment (if you were sent to your room as punishment when you were a child, then you probably think that’s normal). It would not be unrealistic for a character who’s been abused all there life to go to a friend’s house, and be confused when that friend isn’t slapped after breaking a plate.
Victim will most likely have been brainwashed. Most abusers will tell their victim how harsh the outside world is compared to the place the victim is in now. Or they might blame the abuse on the victim themselves (“Well, I wouldn’t have to whip you, if you learned how to properly clean the house). They may also trick the victim into thinking that they aren’t that bad at all, by buying them expensive things and spending a lot of money (“How bad could he be if he spent $200 on a romantic dinner for us?”).
Abuser will think that their actions are justified. They may think hitting their child is an eye for an eye, because the child’s mother died in childbirth, or they may think they’re fixing the victim.
Victim will tend to make excuses for the abusive behaviors, or think that the abuser needs them. They might say something like, “He lost his mother as a child, so I can’t leave him.” Or, “She’s not abusive, she’s just working through some issues.”
Other characters shouldn’t instantly recognize the signs of another character being abused. Unless another character was also abused, or a child services worker, the abuse should be noticed gradually. Other characters may try to come up with some reasons for the signs of abuse (“He must of got those bruises from play football.”)
Part three: Post Traumatic Stress Disorder, and Recovery
The levels of PTSD: Almost everyone who is part of a trauma will receive some level of trauma depending on the role they played in it and what they’ve been exposed to in their lives. A good example of the levels of PTSD is a school shooting. The students and teachers who were injured by the gunman will probably suffer the most post traumatic stress, followed by those who were in the same room where the shooting took place, followed by students and teachers who were in a different part of the school at the time, or not even at school. A foreign exchange student from a country where shootings are pretty common will also probably be less affected than their peers.
Trauma is a bit like an earthquake. Someone who was closer to the event causing trauma will suffer more than people farther away. But like an earthquake, if your circumstances are different, than you could still get hurt more or less than your neighbors (i. e. how strong you house is).
Trauma doesn’t go away all at once. There are way too many stories where the character is suffering from PTSD and after they cuddle with an attractive character, they’re all better. Depending on the tragedy and person, it should take anywhere from months to years to recover, or they may never recover at all. Therapy can help speed up the recovery, but it’s not magic.
The effects of trauma. A character who lost a child, might become overproctective of the one they still have, or an abuse victim who was locked in a basement might fear the dark. Here is a list of some of the possible effects/symptoms of PTSD:
Flashbacks (often caused by triggers)
Nightmares causing victim to wake screaming or in a cold sweat
Not wanting to discuss their traumatic event or trying to forget about it
Substance abuse (often as a method of forgetting)
Feeling emotionally numb
Avoiding activities they once enjoyed
Difficulty with relationships
Anger and irritability
Guilt and shame
Cutting and other self-abuse
Being easily startled
Triggers: A trigger is something that causes someone to recall their traumatic events. Like a trigger for a rape victim might be rape jokes, or seeing their rapist or someone who looks like their rapist.
Trauma should be looked down upon in certain circumstances. Before WW2, people with PTSD were just told to, “Get over it.” If your story does not take place in pretty modern times, other characters should be less willing to comfort them.