The Locked Library Note

A threatening note was slipped under the head librarian's office door overnight. The note is written in a simple code, and investigators have narrowed the suspects down to four library staff members. Each suspect is known to use a distinct personal cipher when passing secret messages.

The rules

1. Four people in this building are known to write coded notes. Each has a different cipher style.
2. Six clues describe their habits and one important property of the note itself.
3. Use the clues to decode the message and name the encoder.
4. One submission. You can’t change your answer.