A janitor is an employee in the same way that everyone else is an employee. A good janitor is one who is punctual and professional. Arriving to work on time and with a positive attitude is the mark of a good employee, no matter what profession.
A good janitor knows his/her responsibilities. Knowing what is expected of you, coupled with a working knowledge of cleaning supplies/methods is essential.
A good janitor always does the job right and never cuts any corners in cleaning. When cleaning something, a good janitor is always one that will not rest until it is 100% clean.
A good janitor displays the professionalism expected of an employee and the willingness to be thorough. A good janitor knows how to clean, what to clean it with, and how best to clean it.