Coloth

Coloth was a young Ulk-Ra.

As an Ulk-Ra
Coloth grew up on a planet home to his people, the Ulk-Ra. He had an older sister named Otchon, who once let Coloth taste reed wine.

Tasked by the chief male of his clan, a young Coloth underwent a rite to become considered a warrior: he went out into the wild with the goal of killing a beast and bringing back its pelt. However, Coloth was captured by Ossu-male, an Ulk-Ra augmented by the War Lords for the War Games and returned to Coloth's homeworld by the Time Lord High Council. Coloth was kept trapped in woven vegetation for a period until — on orders from the War Lord programming — Ossu brutally killed Coloth by slicing him open with his paws. (PROSE: War Crimes)

As a cactus-skinned boy
Coloth was later resurrected, remembering that "[his] family, friends, and homeplanet [were] perfectly fine, but [he had] died a horrible death". He claimed that this was a "long story". (PROSE: Sonnenblumen) He became a member of the Plume Coteries, becoming friends with his birdhemoth steed Rich. (PROSE: Sonnenblumen)

In his new life, Coloth had a new form, resembling a cactus-skinned humanoid (PROSE: Cobweb and Ivory, A Farewell to Arms) with yellowish-green "skin". He could manipulate his appearance at will, for example growing flowers on his skin. (PROSE: Sonnenblumen) Aesculapius was aware that he was an Ulk-Ra but also a "shape-shiter". (PROSE: Life After Death)

In his new body, Coloth, a "cactus-skinned boy", represented the Plume Coteries on Terra Primagenia at the unveiling of Cernunnos. Fleeing from a painted warrior during a praxis trip, Avus shoved past Coloth, and was hurt by his cactus skin. (PROSE: Cobweb and Ivory)

Still a "cactus-skinned boy", Coloth once flew beyond his native universe on Rich's back. As they were passing through the 10,000 Dawns, Rich's wingbeat caused a "rogue wave of dimensional pressure" which led to a Galaxy Accident befalling the Sun of the beings of light's Dawn. Realising the subsequent plague of Invisible Infernos was somehow their fault, the two stayed in the Dawns, helping Dawn to deal with this crisis as well as others, including the destruction of Dawn 10000. (PROSE: Sonnenblumen)

As a cactus-skinned man
Now a cactus "man" rather than a boy, Coloth once walked through a section of the Labyrinth resembling a regular alley. Nezf, Wade, Tabrenilsodvoravitas, and a Shift passed Coloth on their way to steal the Greater Key from the Fortress. (PROSE: A Farewell to Arms)

The adult Coloth was one of the individuals who showed up to Aesculapius's funeral in her Factory of Crystal, secretly a ploy by Aesc (who'd been reborn in a new body) to find out who had sent the trapped parcel which had taken her previous life from her. Coloth nearly bumped into a highly ominous man with a black goatee, dressed in all black with crimson robes, whose every word seemed to be a double entendre implying that he was the one who'd killed Aesc; however, it later became clear the goatee-sporting man was an old friend of Aesc's called Steve whom she seemed to trust completely despite his villainous stylings. Aesc deduced that her killer was instead an old acquaintance in a new body; she specified that even though she was looking for a shape-shifter, she did not suspect Coloth, who was relieved to hear it. Instead, the culprit turned out to be Meistras. (PROSE: Life After Death)

Behind the scenes

 * Prior to Coloth's cameo appearance in Nate Bumber's Cobweb and Ivory almost 20 years after his debut in War Crimes, Bumber contributed a story foreshadowing Coloth's reappearance, titled The Library At the End Of the Universe: A Comprehensive History, to the 2017 fan anthology Shit Trips 2. The story features the Thirteenth Doctor and a boy named "Callum" in the Plume Coteries' library preparing for Coloth's arrival, with the Doctor at one point saying, "History has been broken. The most important person in all of time and space was killed before their time, and they'll be coming through here any day."