Bill Mulder first appeared in "colony". He seemed very stiff and sturn. When mulder went to give his father a hug, bill put it off with a handshake. His un afectionate behavior carried on in "end game". In this episode his sister "samantha" comes back from her aparent abduction. After fox loses his sister in a trade for her life for scullys. he has to tell his father that samantha is lost again. It turns out to be a clone instead of his real sister.
In "anasanzi" we learn that Bill and The cigarette smoking man are on first name basis with each other. It seems that Bill know more than he tells mulder. and perhaps knew about the abduction of samantha before it happened.
In "talithia cumi" Bill is on the brink of telling mulder the truth witch has illuded him so many times before. but right before he tells, he is killed by alex krycek.
