What you say to oneself (yourself), you decide to write down. Whether this is in the form of a journal, diary, or blog, you feel the need to record these important thoughts in some way.
You need to protect what you write down and record for yourself with a key (き). These are your special thoughts and feelings, you don't want anyone else to see! Lock them away with a key!
Currently there are no similar looking kanji which exist in the WaniKani learning system