On Giving Your Characters Flaws & Weaknesses
Today it's pretty common knowledge that part of creating a well-rounded, non-overpowered character involves giving the character flaws and weaknesses. However, many people are still unclear on how to do it properly.
Far too many people treat adding flaws and weaknesses their characters as a way to earn points they can use to purchase "cool" things like talents, skills, and powers. For example, giving the a brilliant biochemist a shellfish allergy and a cranky disposition gives one enough points to essentially "earn" the right to give the character the ability to read minds, too. But this is not how flaws and weaknesses are supposed to work, and if you treat them thus it's probably going to backfire on you.
Flaws and weaknesses only count when they make it harder for the protagonist to realize xir goals and ambitions. For our brilliant biochemist, a cranky disposition and shellfish allergies wouldn't count as flaws and weaknesses until they actually created problems for the character. Perhaps that bad attitude resulted in xir losing out on a contract that would have allowed xir to realize a project xe'd been wanting to complete for years. Maybe the shellfish allergy resulted in a trip to the hospital that caused the character to miss out on a big breakthrough. (This is why being clumsy doesn't count as a flaw for Bella Swan - she doesn't have to work to make Edward notice her in a good way despite being clumsy; in fact, it's just the opposite - he finds her clumsiness endearing.)
It must also be noted that there are several ways that flaws can potentially make people detest a character, and if your character is supposed to be sympathetic you must take them into account.
First, if your character's "flaws" don't actually cause problems or have consequences for your character when they should (EG, a character with a temper who frequently shouts at people yet never truly angers, hurts, nor drives away anyone who matters, or never faces disciplinary action when others would), people will find the story implausible and character to be writer-spoiled.
Secondly, if your character has no traits that people can sympathize with, relate to, or find appealing or admirable in some way, they'll simply find your character repugnant and repulsive. Potentially, flaws can be relatable - almost anyone who's ever worked in customer service will feel sympathy for a character who loses xir temper over a fussy customer with an entitlement complex. That said, someone who can't relate to the struggle of trying to stay calm while dealing with someone who apparently won't be pleased probably won't be affected thus. However, if the character shows other, more positive traits like being funny, caring, or passionate about a good cause, even people who can't relate to that particular flaw can still find reasons to care about and root for that character.
But beware: If the flaw is sufficiently repugnant and impossible to overlook, it may not matter what other positive traits the character has. For example, if someone who torments other people in genuinely hurtful ways for fun (especially people who have done nothing especially horrible themselves), or seriously hurts people over petty grievances and sees nothing wrong with it, or has an extremely bigoted or hateful view of a certain demographic, it may not matter how many good or admirable traits your character has otherwise. If you have a character like this, then either roll with the fact that your character probably isn't really that sympathetic and write accordingly, or alter/develop your character so that these flaws aren't so massive.
Remember: One of the worst things you can do is try to make excuses for your character's flaws, especially if these flaws end up hurting other people. It's one thing to explain them or show that they're the result of some hardship or wrong committed against the character in times past, but if your character's rash and reckless behavior that came about because xe was ignored by xir father as a child ends up getting people injured or killed as an adult, don't expect audiences to overlook it and let your character off the hook just because Daddy was a jerk. Likewise, if your character ends up hurting or killing people because xe has an uncontrollable dangerous superpower, the fact that your character didn't mean to hurt anyone shouldn't be used as an excuse for your character to wander around populated areas freely. Indeed, wandering around in a populated area knowing full well that someone could get hurt or killed if one's powers went off would be criminally negligent behavior, and if someone actually got killed then the character would be guilty of negligent homicide.
Also, any trait that can cause your character difficulty and get xir into trouble is a potential flaw. Flaws and weaknesses need not come from traits that are inherently negative. They can even stem from "good" traits - after all, too much of a good thing can be a very bad thing indeed. Maybe your character's unbreakable loyalty to xir friends leads xir to steal from a needy person because a friend claimed xe needed the money. Maybe your character's "strong sense of justice" leads xir to take out vigilante justice on someone who wasn't really that bad, or was even innocent. Maybe your character works so hard to make everyone happy that xe ends up making nobody happy while wearing xirself down into a frazzle in the process.
So, in summary: Flaws and weaknesses only count if they actually work against your character and genuinely make xir life harder, and any trait or characteristic that works against your character can potentially be a flaw or weakness. Never make excuses for your character's flaws if they end up hurting other people, and be aware that if your character's flaws hurt other people and no genuine change in behavior for the better ever comes about, your character risks becoming an unsympathetic figure very fast.
You may also be interested in:
Character Flaws & Weaknesses Generator
Writing Character Profiles & Bios - Tips & Advice
Building Better Backstories - Tips & Ideas
On Writing Sympathetic Morally-Ambiguous Characters
So You Want To Have A Powerful Or Talented Character Who Probably Won't Be Perceived As A Mary Sue?
Things About Skills, Talents, & Knowledge Writers Need To Know
Character Creation & Development Theory (Or, How To Make Characters 101!)