Strong Characters and Dark Plots

My friend asked me how to make strong characters, and I thought I'd might as well answer it on here. This is what works for me:

Base them on a real person. A lot of times, I base my main character, at least, on someone I know, including me. Ehra in Sarea for example. She's based on two real people: one friend for personality, and another for looks. If you think you'll get too attached to a character you're going to have die, then...I guess you could make up your own personality or something. I don't usually kill my characters, so I'm not much help there. I also base my bad guys on unsavory characters I know, or even historical ones (such as Sisera from Judges 4, or Judas Iscariot from Matthew, Mark, Luke, and John). Basing them on real people helps when you aren't quite sure how your character would react in a situation; simply have them act like the person they're based on would.

Put some of their background into it. Add something more personal about them, so the reader really feels that they know the character. I added a few pages in Sarea when Ehra is talking to her parents, whom she has a very close relationship with. I also added a chapter when Ehra and her sister are chatting with some gossipy village girls, showing how different they are.

My friend also asked how she could keep her plot from being so dark and gloomy. My suggestions are:

Add little children into it. They're so innocent and funny; it lightens up the plot right away. I am naturally very optomistic, so I don't tend to have this problem, but I do have it sometimes. Also add some family into it; if your character is an orphan with a sibling who's her mortal enemy, is running away from a cruel slaverwhile unwittingly falling into a trap their malicious sibling has set, it would make for an interesting story line, but it'd be extremely dark and...not very family-friendly.

Any more suggestions, anybody?

Izori