Sean Knight was a public defender attorney who died in the late fifties from a gas leak. He never became aware of his death and continued on in his routine, none the wiser. When Kevin Matchstick was arrested, Sean was assigned as his PD. When Mirth broke him out of jail, he took Kevin out of the building by placing them both partway in the faerie realms, so that normal people could not perceive them. Thus, they were both surprised to see Sean, who held them at gunpoint and insisted that they tell him what was going on. It turned out that as a ghost, he could walk in the faerie realms without noticing; he had never realized that he could do it before.
However, before they could explain, Lazlo attacked with some Redcaps. Kevin and Mirth quickly defeated the Redcaps while Sean huddled against the wall, unable to deal with all the weirdness he was experiencing. Mirth, meanwhile, had figured out Sean's status as a ghost, although it took a while to persuade him of that fact. Ghosts have very little memory, and Sean eventually realized that not only did he not remember dying, he didn't remember living either. Mirth was convinced that Sean was meant to help them. Sean was not so sure, but he agreed to help, even in the face of Kevin's own lack of conviction!
Mirth taught him how to control his ghostly powers. Sean became more assured of their involvement in the Struggle, and verbally sparred with Kevin about his cynicism, his own optimistic/pragmatic view on life as a public defender coming through. They met up with Edsel, who accepted him easily. They went to Kevin's apartment, where a Leanhaun Sidhe (succubus) started to entrance Kevin in another room. Sean wanted to help, but Mirth explained that as males they would both be vulnerable to her. Edsel therefore took care of it. Sean then had a case of the Sight, seeing a Grackleflint ( Stanis) outside. Mirth caught him and brought him inside.
Sean tried to help with the interrogation by lighting a lightbulb over his head in classic police sweat-down style, but Mirth persuaded him to use his fear power instead. This worked quite well, and they got the information they desired: how the Umbra Sprite had been tracking them. It turned out that he could see the glow of Mirth's green magic in the faerie realms. This led Mirth to make an important decision: he would disappear into a computer system and work on erasing the available data about Kevin, and would leave Sean as Kevin's new guide. Sean was new to the Struggle, but had an unerring moral sense and a strategic mind.
Sean became instrumental in their quest. He set up a series of ambushes whereby they fought the Grackleflints and began to figure out their strategy. Then they luckily found a laundry ticket in a Redcap's pants. Sean went to the laundry and, using a combination of his fear power and lawyerly skills, interrogated the person at the counter, ferreting out the information that the Grackleflints and the Umbra Sprite's nest was The Styx Casino.
The heroes all dressed up and went to the casino. Sean checked out the baccarat tables, but was noticed by the Grackleflints, who were disguised as red-headed humans working as pit bosses. Lazlo went up to touch him, but got no reading about his identity, which had never happened before. They thought perhaps he was Mirth, but when they told the Umbra Sprite, he knew it meant that he was a ghost. Sean and Edsel met up again and tried to figure out where Kevin was. They eventually figured out that the pit bosses were the Grackleflints, and caught one ( Piet) and forced him to bring them to Kevin. He was being held above a pit while the Umbra Sprite and Grackleflints stood below on a plank. Edsel broke in and fought the Umbra Sprite and Grackleflints, while Sean looked on through the walls. At one point Edsel was knocked off the plank, and Sean grabbed her and saved her. As they escaped, Kevin was stabbed by Emil's poisonous spur.
Sean and Edsel grabbed a cab and took the unconscious Kevin home. Edsel left to get Mirth, who was the only hope for curing Kevin. Sean barricaded the door, then walked through the walls to go outside. The Sight had alerted him to the presence of two Grackleflints and Cromm Cruich, a dragon. Sean valiantly tried to scare them off, but Cromm Cruich was unaffected, being, as he said, more obscene and frightening than Sean. He tortured him by burning his forehead with his fingers, but Sean managed to get him back by breaking a lamp and sticking his fingers in the socket, transferring the electricity into the dragon, knocking him back. Unfortunately it wasn't enough, and the dragon blasted him with fire, permanently dispersing him. In this way he came to his final death as he had lived the last few months of his second life, defending Kevin. Mirth arrived at that moment and managed to get Kevin out of his apartment just before Cromm Cruich came in, so Sean's sacrifice and holding action saved Kevin's life.
Sean Knight has the normal powers and limitations of a ghost. A ghost is in the process of slipping away from reality, and so their memories are also slipping away. Many, like Sean, don't even remember that they died. Sean's memories were getting worse as the story progressed.
Sean could make himself intangible, although it took him a while to learn how to do it correctly. For instance, he once got his arm stuck halfway through a trashcan lid.
He could also project fear, and was able to do so effectively against Grackleflints. He was not, however, able to scare Cromm Cruich, who said he was more fearsome than Sean.
He was also able to create and channel electricity to a degree. For instance, he was able to light a lightbulb simply by holding it, and was able to channel the electricity from a house lamp through his body and into Cromm Cruich.
He was also able to see and participate in the half-normal, half-faerie world that Mirth took Kevin in when they were in the police station. Normal humans can't perceive this reality, but Sean could. This ability also gives him a mild version of the Sight, which is basically a form of clairvoyance or precognition that allows him to perceive unusual situations that arise from the Faerie realms.