Clement McDonald, simply known as Clem - or Timothy White by the staff of the Digby York Hospital - was one of the twelve children offered to the 456 in 1965 and the only one not to be taken in that exchange. He would grow up in secret, until Torchwood Three took notice of him when he began chanting the mass message spoken by the children of Earth in 2009.
First encounter (1965)
Clem was born in Scotland in 1953. An orphan, he grew up at the Holly Tree Lodge. In 1965, an alien race designated the 456 came to Earth to offer the human race the anti-virus to a new strain of Indonesian influenza which would wipe out millions of lives, in exchange for twelve children to be, unknown to them, harvested for certain chemicals that gave the 456 an equivalent of a high from Earth narcotics.
In November of that same year, the British government and the Torchwood Institute chose Clem and eleven others from the orphanage, believing that that no one would miss them, and so kept the act a secret. After arriving in a secluded location, Captain Jack Harkness drove them to a white light, and guided them from there. (TV: Children of Earth: Day Four) However, Clem was the only one who managed to escape. He was chased for several miles until the 456 decided to stop. (TV: Children of Earth: Day One)
After that, he was somewhat connected to the aliens, and possessed a sense of smell far beyond normal for a human being, as well as psychic ability. This also left him with a consistent verbal tic; "Isn't it?", accompanied by a twitch of his face. It was the result of a suppressed traumatic memory of the night he was bused to the 456 and asked, "It's safe though, isn't it?" to Jack before stepping into the light that appeared before him.
For the next four decades, he was haunted by Harkness' face - to the point where he recognised Harkness's scent years later - and was, for unknown reasons, submitted to a psychiatric hospital in East Grinstead, after spending most of his life in and out of care in Leeds. He subsequently lost his Scottish accent and went under the alias of Timothy White to avoid detection by the government. (TV: Children of Earth: Day One)
Gwen theorised he was spared because he was on the cusp of puberty, which may have rendered him useless to the 456. Although they seemed to have simply abandoned him, some measure of a two-way connection did remain. (TV: Children of Earth: Day Four)
Second encounter (2009)
In September 2009, the 456 returned and began transmitting a message through the voice of every child on Earth: "we are coming." Clem also chanted the words, the only adult known to do that. This attracted the attention of Gwen Cooper from Torchwood Three, who visited him on the first day of the 456 incident. As he listened to Gwen, he used his sense of smell to determine she was telling the truth and that she worked to stop alien threats. He told her his actual story and that he could sense the coming arrival of the 456.
He also gave Gwen a stunning pronouncement: she was three weeks pregnant. In shock, she left. Later, after being discovered by the British government, he was targeted for assassination. When agents arrived at the hospital to terminate him, he used his precognitive knowledge to take action, attacking his interns and escaping into the night. (TV: Children of Earth: Day One)
On the second day, he was seen in a pub near London, where his smell sense of the 456 grew even stronger; the people in the pub grew suspicious of him. When the children began chanting that "we are coming tomorrow," Clem once again joined them and was horrified by what he was stated to have said afterwards. Ranting that he could smell the 456 getting closer, Clem's panicked state made everyone nearby think he was crazy. After the barmaid determined he had stolen a wallet, she immediately called the police, who arrested him after a brief struggle. (TV: Children of Earth: Day Two)
On the third day, he was released on bail by Gwen and Andy Davidson, and was brought to their makeshift base in London belonging to Torchwood One, after their hub was destroyed by the government. To his utter horror, however, he found himself facing the demon of his nightmares — Harkness, who hadn't changed a bit in 44 years. (TV: Children of Earth: Day Three)
In revenge and anger, he shot Jack dead, but was further stunned after he revived. Gwen was able to calm Clem down and he was able to provide some assistance to the team again. Jack and Ianto Jones were able to get into Thames House to negotiate with the 456, who had arrived the previous day and demanded 10% of Earth's children. When the 456 refused to go empty handed, it attacked Clem with a hypersonic signal; the trauma causing internal bleeding to his body from his brain. Eventually, before it killed everyone inside Thames House, it "disconnected" him; effectively killing him in front of Gwen's eyes. (TV: Children of Earth: Day Four)
The next day, while trying to figure out a way to fight the 456, Jack speculated on why the 456 killed Clem since he was no threat to them. He postulated that Clem's connection with the 456 hurt them in some way and decided to use the signal that the 456 killed Clem with against them. The signal apparently killed the ambassador and forced the retreat of the 456. (TV: Children of Earth: Day Five)