Everyone has secrets. Toph has a few more than most people.
If anyone ever tells you that they're completely open, that they've got nothing to hide, then that person is a goddamn liar and you should keep your eye on them twice as hard as anyone else. Everyone keeps secrets, everyone lies, and everybody has something to hide.
Some people have secret pasts, or criminal lives, or illicit romances to conceal. Most people's secrets are far more more mundane; a secret infatuation with a coworker, perhaps, or maybe a hidden desire to 'play for the other team'(so to speak). But regardless of who they are or where they came from, they are united in their need to be secretive.
Take, for example, the case of one Toph Bei Fong. She's far from the most social kid you'll ever meet, and she'll never be Miss Talkative. She has her secrets, and she likes to keep them. But if you got to know her, and were talking and you were willing to share a secret of your own, you might be able to pry a story or two out of her.
With a bit of effort, you might find yourself hearing some stories of her life as a blind girl, before she learned to see with her feet. She was helpless and a little gullible back then, and several of the 'friends' her doting parents inflicted on her in a vain attempt to make her life as normal as possible were very cruel. It could be you'll hear an amusing tale or two.
If you manage to befriend her and really work at it, she might, for instance, tell you the story of the creep she bumped into on her way home from her first Earth Rumble tournament, and what he tried to do to her (although no amount of cajoling will reward you with exactly where she left his beaten body afterwards. If you ask, the most you'll get is her knowing smile, and the distinct impression that you're being left out of a funny joke). But there's one story she'll never tell, no matter how much you beg, no matter how much you mean to her, and no matter what you offer to do in return. It's the story of the day she discovered how to 'see' through the ground.
Imagine a Toph of...let us say nine years. By this point she'd already endured three years of Master Yu's training, if you could call it that, and was sick to death of basic stances and breathing exercises. She longed to move on to something else, and so one night, as she was about to crawl into her bed, she reached out to the ground beneath her feet and found that it /resonated/. Curious, she pressed against it with her feet all the harder, and reached out once more, harder, with her mind. And as she did, she felt vibrations move outwards from her foot and bounce back from objects around her. And then, amazingly enough, she could see!
Well, not actually /see/. Even at nine years old, she knew enough to tell that this was different from what her parents and servants could do. But even so, a giddy feeling began in her belly and moved upwards, until she could contain it no longer and began giggling with joy. She could /see/! Just wait until she told Mommy and Daddy, she wouldn't be their little blind girl anymore, she wouldn't be their little secret, she'd be able to go out and play and go to parties and they'd love her forever and ever and...
On and on her train of thought rolled as she expanded her 'vision' around her. She found she could see into neighbouring rooms, as the whole mansion was built on the same slabe of rock, and gleefully set about spying on the servants as they worked. She could see Maddy the cook cleaning dishes in the kitchen, and one of the maids she didn't recognize putting fresh towels in the closet. Her euphoria increased as she expanded her range of 'vision'. This was even better than seeing with your eyes; she could see right through walls! There was no secrets from her, not anymore and-
Her heart sank as she saw into her parent's room.
She'll never tell anyone how she watched them bicker. She couldn't hear what they were saying, but she knew it was about her. They were always arguing over her; they thought she didn't hear but she did, behind doors and through walls. Her mother wanted Toph to have more freedom, to live a fuller life, but her father was adamant that she stay at home, where she was safe.
She'll never tell anyone about how she watched as her father, fed up at last of hearing her, drew back a hand and struck her mother sharply about the face. Toph gasped as if she was the one who had been hit, and shrank slightly, one fist pressed against her chest.
She'll never tell about the tears that rolled down her cheeks as she watched her father drag her mother over to the bed, throw her down on it, and roughly take his pleasure of her.
And most of all she'll never tell about the silent oath she swore as she stood in mute witness to the violence, an oath to become so strong that no man could ever hurt her like that.
Some servants walked in and were surprised to find her standing in the middle of her room, in her bedclothes, staring blindly at the wall and crying softly. No matter how they asked, she refused to say a word to them about it. Even after her parents questioned her on it the next day over breakfast, she refused to speak on the matter. What she'd seen and what she swore were her secrets now, and she would never tell anyone.