First appearance: Welcome to Lovecraft #6
Use: View, edit, or remove memories from a person
Door: Most people’s heads
The Head Key allows the user to open someone’s head so that thoughts and memories can be inserted or removed from the subject’s mind.
Thoughts and personality traits seem to be removable, as Miranda and Kinsey Locke removed their fears, becoming much more confident afterwards.
The Head Key can be used to “share” memories between people by taking them out of one person’s head and placing them into another.
Using the Head Key, items and knowledge can be directly placed into a persons head and later retrieved.
The memories themselves are not always accurate, and can be affected by the subject, particularly their state of mind. For example, Bode Locke’s recollection of the Lady in the Well was inaccurate.
Though many memories seem alive, they are seemingly invulnerable to dying, or at least drowning
As Erin Voss was being completely emptied out, her mental state degraded and her hair became white.
The keys have worked differently for Rufus Whedon, including the Head Key which Zach Wells attempted to use on him; not allowing him to alter Rufus’ memories.
Source: Locke & Key Wikia