William 'Bill' Mulder was the surrogate father of Fox Mulder, biological father of Samantha Mulder and husband of Teena Mulder. An employee of the state department for much of his life, Bill was involved in both Operation Paper Clip and attempts to create Alien-Human hybrids before the meeting with the Colonists at El Rico Air Force Base. Following this, Bill was forced to surrender his daughter and became a member of the Syndicate. He was the lone dissenter of an agreement which seemingly ensured that Colonisation would begin in 2012. Bill spent the rest of his life ruined by guilt, and confessed some of his sins to his son in 1995 before being shot and killed by Alex Krycek.
- Everything Dies: Part 1 & 2