Adric

Adric was a companion to the Doctor in his fourth and fifth incarnations. He grew up in another universe and was a stowaway on the Doctor's TARDIS before he was formally welcomed. He died trying to stop a ship from crashing into Earth. The explosion which destroyed the ship and Adric ultimately led to the extinction of the dinosaurs. (DW: 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 gold star badge 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 onboard the TARDIS when it took off. To prove himself to the gang, he tried to steal some riverfruit, but failed. (DW: Full Circle)

E-Space adventures
Adric hid aboard the TARDIS until 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. (DW: State of Decay)

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

The Doctor assembled another version of K9, who joined them on their journeys. (DWAN: Inter-Galactic Cat, Conundrum, Planet of Paradise, Plague World, Just a Small Problem) At some point prior to the Fourth Doctor's attempt to fix the TARDIS' chameleon circuit, (DW: Logopolis), the Doctor dispensed with this newer K9.

Adric and the Doctor encountered the Master again shortly after leaving Traken as 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 once again defeated the Master, the Doctor was badly injured. All three companions watched him regenerate. (DW: Logopolis)

With the Fifth Doctor
Immediately after the Doctor's regeneration, Adric was kidnapped by the Master and held 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. The Master seemed to be destroyed in the city. (DW: Castrovalva) Adric's relationship with the Doctor deteriorated after the Fourth Doctor regenerated and Tegan and Nyssa began travelling in the TARDIS. (DW: 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. (PDA: Divided Loyalties, DW: Earthshock )

Adric was often frustrated by his clumsiness and fiercely competitive. 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 with Briggs freighter.He gave his life by remaining on board the crashing, time-travelling ship. With Adric onboard, it crashed into prehistoric Earth, causing the extinction of the dinosaurs. (DW: Earthshock)

Legacy
Just before he regenerated, the last word the Fifth Doctor said was "Adric?" (DW: 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. (IDW: The Forgotten) His death causing the extinction of the dinosaurs could mean adrics death was a fixed point in time and ultimately adric was one of the most important people in creation as he allowed mammals to take over which would allow humans to evolve and they would go to become a major space power.

Alternate history
In an alternate version of these events, after interference by the Fifth Doctor in an attempt 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, and accompanied them back to the Victorian era. Adric finally died after helping the Doctor recover his TARDIS from Thomas Brewster. (BFA: The Boy That Time Forgot)

Personality
Adric was a mathematical genius. He was proud of his Elite status. He wished be be accepted by his brother Varsh's gang, the Outlers, and tried to steal fruit to prove himself. When his brother died, he kept his belt, a symbol of Outler membership. (DW: 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. (DW: State of Decay) Aukon sensed Adric had the strength of spirit to survive becoming a vampire. (EDA: The Eight Doctors) He ultimately gave up his life by remaining on board Captain Briggs's freighter to prevent it from crashing into the Earth. He died holding his brother's belt. (DW: Earthshock)

Adric was fiercely loyal to the Doctor, and enjoyed a good relationship with the Time Lord, who acted as his mentor. Adric was brave; for instance when defying the Master (DW: The Keeper of Traken) and when he gave his life to save the Earth. As he crashed to his death, unable to access the computer system to input his last mathematical calculation, he mused, "Now I'll never know if I was right." (DW: Earthshock)