The antonym for simple is complex.
When we describe something as simple, we refer to it as being easy to understand or not complicated. On the other hand, complex refers to something that is made up of many interconnected parts and is not easy to understand. For instance, a simple math problem might involve basic addition, while a complex one could involve multiple steps and various mathematical operations. So, when looking for a word that conveys the opposite of straightforwardness, complexity fits perfectly.