Neil Hilton was a companion of the Seventh Doctor and a space-time anomaly. He had a traumatic experience as a child which led to his eventual encounter with the Doctor.
On Wednesday 31 March 1982 when Neil was still a child, he and his friends used an ouija board. During this session one of his friends, Andrew, became so scared that he ran out of the building and was later hit by a car and killed.
Decades later in March 2007, after a system shutdown at his work gives Neil the opportunity to take a breather and go for a smoke break, he learns of a seance being held and decides to attend with several of his co-workers. At the seance, the Ouija board spell out his name which causes Neil to become greatly distressed. In the midst of the chaos, the Seventh Doctor arrived at the seance and invited Neil to travel with him and Ace.
Neil travelled in the TARDIS for years, during which time Neil learned from Ace that then Doctor was trying to exorcise him and make him the best possible version of himself. On one trip, the Doctor took Neil to Friday 31 March 2007, and showed Neil his own death. Neil was supposed to die in a hit and run accident instead of attending the seance. The Doctor made him understand that one day he would be returned to his own time with foreknowledge of his death, and that it was his choice what to do with that knowledge.
Neil wrote a blog about his travels with the Doctor and Ace, specifically to catch the attention of his old friend, Tim Leicester. Neil sought to teach Tim about choices as the Doctor had for him. Neil was aware that each time the TARDIS landed it could be his last trip. He wondered if he could go on travelling forever or if one day he would be killed during one of his adventures. He also noted that he could make the choice not to attend the seance and instead die by the roadside with the Doctor by his side. This resulted in Tim changing his life around and convinced him to quit smoking, greatly extending his life in the long term as the Doctor and Ace later discovered.
Neil eventually departed the TARDIS, and Ace wondered if the choice he made had been the right one. The Doctor reminded her that all choices are the rights ones if you consider all the consequences, and reminded her that as she can still remember him, he will will be just fine. (PROSE: Séance, or Smoking is Highly Addictive, Don't Start)