The Gatekeeper is a unique boss that appears in Scene 7-1. Just as it's name implies, it's task is to guard Dark Demon's fortress from intrusion. Heather puts the keys from the Keymasters Headdy fought into a bag located on the back of the machine while the hero distracts it, so the five keys can be used.
Once the keys are in, energy fills the sky and the illusion breaks like glass, revealing Dark Demon's menacing fortress. At this, The Gatekeeper loses it's marbles and captures Heather, promptly turning into the Nasty Gatekeeper. It's weak spot is the eyes or face.
The Gatekeeper appears as a tall puppet or robot, standing even taller than our puppet hero. It has a light, metallic blue coloring, mostly prominent on it's legs, chest and head. Two antennae stick up from the left and right sides of it's spherical head, an odd green device in the middle of these antennae. Instead of a face, a screen is used instead; When aggravated, this screen slides upward, revealing an angry set of yellow eyes.
A pink plate lies at the base of it's neck, this same pink color adourning it's arms, waist, and the joints for it's legs (these leg joints appear to either stretch or inflate during movement.) The joints for it's arms appear to be bolts of some kind, which don't appear to move when it's arms do. On it's chest are several gadgets of sorts; A slot, a gauge, and two yellow buttons or lights. The aforementioned gauge goes out of control when the Gatekeeper does.
The arms are mostly pink, with green bands appearing where wrists would go. Instead of hands, two sets of nubby wrecking balls are used, mostly for smashing intruders. The legs are lengthy and thin, with pink spheres for knees and stubby feet; These feet are waggled whenever the Gatekeeper takes to the air, and aren't really seen for tasks such as walking, as the legs do not appear to be structured for such an activity. Upon it's back is the "bag" used to open the path to Dark Demon's fortress.
In the American version of Dynamite Headdy, the Gatekeeper simply attacks Headdy and Heather, whilst the latter tries to throw keys in it's bag. If she makes it, she cheers and readies the next key. If she misses, the key bounces away, and she has to frantically chase it and try again.
In the Japanese version, the Gatekeeper praises the puppet hero for all of his hard work, seemingly grateful. Then, it implores him to leave, as apparently the location they are in isn't Dark Demon's fortress, and he would be putting everyone in danger. Heather appears and calls this out as fraudulence, as she appears to know the secret the Gatekeeper is trying to keep. This surprises the machine into attacking them.
With both versions, the Gatekeeper attacks by hopping around the arena and using it's arms for various attacks. When it enters it's psychotic phase, it's arms become lengthy, stretchable, clawed and more dangerous. During this phase, the gatekeeper grabs Heather, and later Bino, to use as a human shield. Hitting the latter grants the player a Secret Bonus Point, whilst hitting the former takes off some of Headdy's health.
- In the Japanese version, the Gatekeeper is a female, rabbit-eared Geisha named Yayoi, with a pink dress, with green in the middle and yellow lacing, she also has black hair. The Nasty Gatekeeper is also a Geisha, called Izayoi. Izayoi has a terrifying face that resembles a ghost. There appears to be a distinct difference between the two; According to what is known, the two geishas are sisters.
- This is the only boss, aside from Mad Dog, that can get Beau off the screen, but it is mostly Headdy who can do this, and it is completely optional.