ONE. what counts as a flaw? [ what's a mary sue/gary stu? ]
Mary sues and gary stues are perfect characters and they can never do anything wrong. As for flaws, make sure it's something bigger than "trips a lot"/ "very clumsy" or any other gimmicks. For example, a good flaw would be insecurity- sure it can be annoying, but that's what makes us human, our sometimes annoying flaws.
Admins will be sad if your only flaw is tripping.
TWO. what are the sectors? and what do they do?
There are four different sectors (Ignis, Aqua, Ventus, and Terra). Each of them consists of people who can control one specific element. You are assigned into a sector based on which element you have mastery over. Ignis is fire, Aqua is water (duh), Ventus is wind, and Terra is earth. Read this announcement for more details.