If you're talking about acting or feigning:**
* Real
* Genuine
* Authentic
* Truthful
* Sincere
If you're talking about imagining or fantasizing:**
* Reality
* Fact
* Truth
Example:
* "He pretended to be sick, but he was actually healthy."
* "She was pretending to be a princess, but she knew it was just a fantasy."
Ultimately, the best opposite for "pretend" will depend on the specific meaning you want to convey.