Playing with fire, you are, Xar. Gonna blatantly public shame now.
Do Not:
Excessive length, detail, or complexity isn't always good. It can make a profile tedious to read, and sometimes simple and concise is much better and appreciated. Sometimes it may be necessary though and that's fine; just don't overdo it.
Do Not:
Avoid making characters that are cliche, tropey. or 'dark' if possible. It is incredibly difficult to make dark backgrounds sound good and I'd say most people can't do it (including myself). One of the problems with dark histories is that they tend to go into specific detail specifically for the dark parts, and that's probably one of the biggest problems that makes them a turn-off. This can be solved by making events simple and concise. For example,
another character of mine had an event happen that mentally scarred her and made her weary about working with other people. In previous iterations of the profile, I went into the details of the event that happened, but for the current profile, I entirely cut it out and replaced it with something that was basically "Yeah, this event happened and this was the result" and it made it sound
much better (reading it again, I didn't even mention what the event was when it happened that's how vague it was; it was only after the fact that it happened that I touched on it a tiny bit).
Do: As Leycil just happened to say
you sniping son of a-, keep things unique and different. Too much of something special makes it no longer special.
Anyway, it'd probably be more helpful to plaster this onto a new character guide or something.