She's twelve the first time Trinity disappears. She can't remember a time when Trinity wasn't around, but she survives and Trinity shows up a week later.
She leaves again, for longer. Faith wakes one morning, and she's gone. There's a note on the table and food in the fridge and what's left of her Trig homework completed in her schoolbag. (Trinity's cool like that.)
And so it goes, with Trinity sneaking out in the middle of the night, and once, just once, sneaking in with a guy whose voice she doesn't recognize. She stays awake, but they do nothing but talk and she drifts off wondering where she's heard the name Morpheus before.
She's sixteen the first time she manages to follow her, watching from the window, climbing down the fire-escape, trailing two blocks behind until she arrives at a dead end. A deserted dead end. She backtracks, looks for a ladder,a concealed door, anything, but there's nothing to explain Trin's disappearing act. Just a phone box with the phone hanging for its hook near the alley entrance. She's gone more than a month, after that, her longest ever, and Faith can't help feeling that it's her fault, that Trin somehow knows. She doesn't try following her again.
(Her trackers don't work either, no matter how many she makes and steals and plants in Trinity's clothes.)
Which is probably the reason Trinity sits her down one day, after throuaghly debugging the house and smuggling sunglasses guy in through the back window, and tells her how her world is a lie.
She may be good, but Trinity is better, so much better.
Trinity's a superhero. Faith's always wanted to be like her.
So she takes the red pill.