Functions are close, but the real benefit of the behaviors in Stencyl is that they are exactly like the behaviors in C2. They are cross application, you update one codebase and "if you recompile" your app then it will get the updates. But the key difference between the two Stencyl and C2 is that in Stencyl you can create them on the fly and save.
For Example: Create Behavior, add a bunch of events (much like you functions), Save. then you just insert it into your apps.
Take C2 Functions and add a Save as Behavior option that would be great. Then you can Insert that Behavior into any app you build.
I apologize for mentioning Stencyl so much, it is a decent tool but I dropped it because the out of box examples were overly complex and were not reproducible, the out of box behaviors also didn't consistently work. As compared to C2, so far I have seen everything work as expected and am very impressed, the UI could use an improvement, but I am sure that will come with time.