|Spoiler Alert: This page contains spoilers. Read at your own risk.|
|Single Target, Multiple Targets|
Gatekeeper (ゲートキーパー) is a Magic Sequence that's constructed in the Root while going through the Gate located between the subconsciousness and consciousness. The Gate is exposed in the Idea and Gatekeeper scrambles all Magic Sequences coming out of the Gate. Unless unlocked by Gatekeeper, a magician can't use magic. 
Shiba Miyuki made Jasmine Williams and James Jeffrey Johnson believe that Gatekeeper is a "secret technique" of the Yotsuba Family to deflect attention away from Tatsuya, but in fact it's actually his "secret technique".