Board Thread:General Wiki Discussion/@comment-27023843-20160713121846/@comment-5614678-20160821074125

Slenderman doesn't usually fight people. He either seduces them or just messes with their head. That's why they call him "The Operator." He's a middleman between you and the guy who actually kills you. But there are multiple interpretations of him, and the core heart of the character is that we really have no idea what he is.

Jeff the Killer doesn't seem to have a dominant interpretation like The Operator does. Depending on which story you're reading he's ever a crazy kid with a knife, a crazy fully-grown man with a knife, or a crazy kid/man with supernatural powers. According to Seusser, however, Jeff is a supernatural killer with the power to teleport wherever he is summoned and unknown dark abilities like Slenderman.

So, the answer is: Jeff would try to fight Slendy, but Slendy would probably just escape or try to hide away. Jeff would probably end up having to fight it out with a bunch of Slendy's proxies, and then Slendy would disappear.