Rokurokubi are a type of oni (demon) with the appearance of ordinary women. They may even marry and live completely blameless lives during the day. At night, however, they reveal their true nature-- their heads rise as their necks stretch to impossible lengths. These snake-like necks allow the heads to swirl about and appear around corners when least expected.
Which is what the rokurokubi enjoy most of all. Being oni, they love to scare ordinary mortals. They also like to spy, which is only natural when your neck allows your head to roam independently of its body. You can probably find all kinds of weird hiding places for eavesdropping or windows to peer into and find out everything there is to know about your friends and neighbors.
I'm not sure how dangerous rokurokubi are, or if they have any supernatural powers beyond stretching their necks. In the film clip, the rokurokubi doesn't actually do much to either of the men. She just swirls around on her neck stalk, cackling, and they keel over from fright. I thought at one point she was going to go anaconda on that one jerk-ass and squeeze him to death, but no.
A missed opportunity.
Sure, rokurokubi enjoy shocking people. But I've known non-rokurokubi who delighted in doing the same and with much less justification. They certainly seem less frightening than Kuchisake-onna, who wants to kill you. Perhaps rokurokubi depend primarily on surprise or their victim's weak stomach. If you don't have much a problem with a rokurokubi's bizarre appearance, you can probably get away easily.
Or make a new friend. Or hire her to work for your company; she may come in handy in matters of corporate espionage.