1. The problem is to simplify the expression $-i$ where $i$ is the imaginary unit with the property $i^2 = -1$. 2. Recall that multiplying by $-1$ changes the sign of a number or expression. 3. Since $-i$ is simply the negative of $i$, it cannot be simplified further in terms of real numbers. 4. However, in the complex plane, $i$ represents a 90-degree rotation counterclockwise, so $-i$ represents a 90-degree rotation clockwise. 5. Therefore, the simplified form of $-i$ is just $-i$ itself. Final answer: $-i$