Adric

Adric was a companion to the Fourth and Fifth Doctors. He grew up in another universe called E-Space. He died trying to stop Captain Briggs' space freighter from crashing into Earth; his failure facilitated the proliferation of mammals and ultimately humanity's very existence. (TV: Earthshock)

Early life on Alzarius
Adric was an Alzarian born on the planet Alzarius in the parallel universe of E-Space in the 32nd century. He was ranked as an elite and was awarded a blue star-shaped badge with gilt edging for mathematical excellence. His brother Varsh belonged to a gang of teenage thieves. Adric wanted to join them, but was refused. Varsh asked him the embarkation question: "Where will you be when the starliner leaves Alzarius?" Adric replied he would be somewhere else entirely. This would prove to be true; he would be on board the TARDIS when it took off. To prove himself to the gang, he tried to steal some riverfruit, but failed. (TV: Full Circle)

E-Space adventures
Adric stowed away on the TARDIS and was discovered by the Fourth Doctor and Romana. He pretended to share the agenda of the Three Who Rule, a trio of vampires who ruled over a small community. He tried to save Romana from being sacrificed but failed; both were saved by the Doctor. (TV: State of Decay)

After Romana
After Romana left, Adric and the Doctor travelled together. They left E-Space and entered the Doctor's universe. (TV: Warriors' Gate) Soon after, they went to Traken and met Nyssa. They saved Traken from and returned the Keepership to its rightful heir. (TV: The Keeper of Traken)

The Doctor assembled another version of K9, who joined them on their journeys. (PROSE: Inter-Galactic Cat, Conundrum, Planet of Paradise, Plague World, Just a Small Problem) At some point before the Fourth Doctor's attempt to fix the TARDIS' chameleon circuit, (TV: Logopolis), he sent this K9 to Sarah Jane Smith, care of her Aunt Lavinia. (K-9 and Company: A Girl's Best Friend)

Adric and the Doctor encountered the Master again shortly after leaving Traken. Doctor made plans to use the mathematical skills of the Logopolitans to repair the faulty TARDIS chameleon circuit. They met Nyssa again, as well as Earth stewardess Tegan Jovanka, both of whom joined them on their travels. Although they again defeated the Master, the Doctor was badly injured. All three companions watched him regenerate. (TV: Logopolis)

With the Fifth Doctor
Immediately after the Doctor's regeneration, the Master kidnapped Adric and held him in the Hadron web. The Master planned to use Adric's mathematical prowess to produce his own version of Block Transfer Computations. With this power, the Master made the town of Castrovalva as a trap for the Fifth Doctor. However, the Doctor saw through the Master's plan and rescued Adric. Without Adric in the web, the city began to collapse. The Doctor, Adric, Tegan and Nyssa escaped, while the Master seemed to be destroyed in the city. (TV: Castrovalva) Adric's relationship with the Doctor deteriorated after the Fourth Doctor regenerated and Tegan and Nyssa began travelling in the TARDIS. (TV: Logopolis, Castrovalva) Adric believed the girls were a hindrance to his relationship with the Doctor. He also had a clear preference for the Fourth Doctor. (PROSE: Divided Loyalties, TV: Earthshock ) Adric was often fiercely competitive, yet frustrated by his clumsiness as he struggled to find a useful role in the TARDIS crew. He finally told the Doctor he wanted to return to E-Space. After showing the Doctor his computations to engineer the return, Adric admitted he really wished to remain on the TARDIS. He was caught up in a failed plan of the Cybermen to bombard the Earth with Briggs' freighter. He died by remaining on board the crashing, time-travelling ship. With Adric on board, it crashed into prehistoric Earth, causing the extinction of the dinosaurs. (TV: Earthshock)

Legacy
An illusion of Adric appeared to Nyssa and Tegan to dissuade them from entering Kalid's inner sanctum, warning them to turn back or they would destroy him. The girls thought at first this was the real Adric, until they noticed the illusion was wearing Adric's by-now-destroyed star-shaped badge. (TV: Time-Flight)

Just before he regenerated, the last word the Fifth Doctor said was "Adric?" (TV: The Caves of Androzani) Adric's death haunted the Doctor for many years. His tenth incarnation watched Adric die again — this time as a manifestation of the TARDIS Matrix. (COMIC: The Forgotten)

Nyssa named her only son Adric in his honour. He had two elder sisters, Neeka and Tegan. (AUDIO: Heroes of Sontar)

Adric's failure to prevent Briggs' freighter from crashing into the Earth (TV: Earthshock) arguably fulfilled a Silurians' prophesy of the of the Earth's devastation and the extinction of the dinosaurs, which the Moon had to not fulfilled as originally interpretted. The result was the proliferation of mammals and their evolution which ultimately lead to humanity. (TV: Doctor Who and the Silurians, Cold Blood, Dinosaurs on a Spaceship)

Adric's distinction as the Doctor's youngest non-robotic companion on-screen was superseded by Amelia Pond whom the Doctor readily agreed to take with him despite her tender age of seven years. His TARDIS' malfunctioning Helmic regulator, however, caused his five-minute repair flight to return him twelve years later and to then an additional two years later, such that Amy is not known to have boarded the TARDIS until her early 20s. (TV: The Eleventh Hour) Nevertheless, the Doctor's elder self subsequently returned twice to young Amelia, carrying her to bed later that night and visiting her the next morning to tell her to be patient and preview the adventures that awaited her. (TV: The Big Bang, The Angels Take Manhattan) Jo Grant was recognised as a companion long before finally travelling aboard the TARDIS in Colony in Space; and other companions, such as Liz Shaw, Jackson Lake, Craig Owens, et al. never travelled at all. Amelia's record was arguably lost to her own daughter, as the then-future companion Melody Pond/River Song flew aboard the TARDIS in utero as a zygote and embryo (and indeed initially as the two separate sex cells within her parents), being conceived whilst transiting the Time Vortex between the events of The Big Bang and The Impossible Astronaut; (TV: A Good Man Goes to War) the embryonic Melody/River was present although un-seen within the TARDIS in Space and Time.

Alternate history
In an alternate version of these events, after the Fifth Doctor interfered to try to recover his TARDIS, his sub-consciousness helped Adric break the final logic code on the Cybermen's computer. This let Adric land - and survive - on Earth. Adric created a City of Excellence and ruled over a sentient race of scorpions which evolved in place of the dinosaurs. Adric created this world with Block Transfer Computations. Five hundred years later, Adric met the the Doctor and Nyssa. He accompanied them back to the Victorian era. Adric finally died after helping the Doctor recover his TARDIS from Thomas Brewster. (AUDIO: The Boy That Time Forgot)

Personality
Adric was a mathematical genius, but was socially awkward. He was very proud of his Elite status. He wished to be accepted by his brother Varsh's gang, the Outlers, and tried to steal fruit to prove himself, but failed. When his brother died, he kept his belt, a symbol of Outler membership. (TV: Full Circle) He pretended to go along with the plans of the vampires in a ploy to rescue Romana, with whom he got on well. (TV: State of Decay) Aukon sensed Adric had the strength of spirit to survive becoming a vampire. (PROSE: The Eight Doctors) He died by remaining on board Captain Briggs's freighter to try and prevent it from crashing into the Earth. He died holding his brother's belt. (TV: Earthshock)

Adric was fiercely loyal to the Doctor and had a generally amicable relationship with the Time Lord, who acted as his mentor. Adric wanted desperately to be brave and accepted; he defied the Master (TV: The Keeper of Traken) and died trying to solve a math/computer problem to save the earth. As he crashed to his death, unable to access the computer system to input his last mathematical calculation, he said, "Now I'll never know if I was right." (TV: Earthshock)