I can think of so many exceptions to this little rule. Let's start with me. The narrator of both of my Shakespeare/baseball novels-in-verse really doesn't have a contradictory nature. But Colleen in the Stoner-series does. She's got a potty mouth and can be sexually indiscriminate yet she's tender-hearted and compassionate.
A pious narrator with a stash of porn doesn't strike me as contradictory while a pornographer with a passionate spiritual life does. The former is predictable, the latter not so much. The former is just somebody with a secret. But not the latter.
Characters in kids' writing really can be flat and (here's a nice synonym for predictable) likely.
And sometimes writers try and round their characters out by making them oxymoronic -- the kind bully, the compassionate gossip, etc. That might work now and then but it isn't paradoxical.
I imagine if we're searching for true paradox we need look no further (thanks for that, Muriel!) than our secret selves.
P.S. The wonderful mare Rachel Alexandra (Horse of the Year) ran second yesterday. I figured the distance would be too much for her, and was right. I cashed an exacta ticket (she ran second) but wasn't happy about it.