Demon
The demon radical and the demon kanji are exactly the same! Wicked!
き
おに
None
How do you keep a demon locked up? Use a key (き). Use a lock and key and keep that demon locked up safely behind bars so he can’t get out and do bad demon stuff.
Picture yourself locking up a demon with a special demon containing key. Hear the key click into the lock as the demon screams to be set free.