Depends on what are you going to do. Technically it should be possible to work knowing only Kotlin, of course, but you'll stumble upon C / Objective-C interoperability quite fast. There basic knowledge of these two languages will be needed. Right now there are two docs on both language interop bindings which describe how e.g. C concepts like pointers, lvalues and memory management are represented in Kotlin, but that's all.
Also you'll probably need some knowledge on underlying platform APIs, like Foundation with friends on iOS / MacOS or POSIX on Linux. We'll definitely see Kotlin wrappers (including multiplatform ones) for some concepts like working with filesystem, so there would be a bit less C in K/N usage, but that's stretch goal and needs some time to appear.