Akka doesn't have native coroutines / lightweight threads, it relies on regular JVM threads. it does reuse them for Actors but it dons't do the same things as Qasar (user level threads via bytecode instrumentation).
I'm a big Scala and Akka fan, but thought it's worth noting.
Akka is a library not an extension of the JVM so it would not be able to provide native features to Java. I didn't know that Quasar did more. One is able to write much better concurrent code with Akka than with vanilla Java but I think you can still run into threading issues where Java makes this unavoidable, like when playing with audio.
Quasar also relies on JVM threads. Akka's lightweight actors are just as native as Quasar's lightweight threads.
(main Quasar author): Akka actors are nothing like Quasar fibers (or even Quasar actors). Quasar provides true lightweight threads - though they're not "native" to the JVM, they're as real as Erlang's processes or Go's goroutines - while Akka actors are a formal way to organize asynchronous callbacks.