Child Master (The Then and the Now)

At some point during the Last Great Time War, the Time Lord once known as the Master regenerated into the body of a child and chose to abandon his name and title, forming a fleeting aliance with the War Doctor and the Squire, until an attempt to flee the conflict resulted in him reverting into his.

Regeneration and independent activity in the Time War
At some point, the War Master (COMIC: Fast Asleep) regenerated into a new incarnation. Now in the body of a small child, and still active in the Time War, they were said to have abandoned the name of "Master", as he simply operated with no name whatsoever. (COMIC: The Organ Grinder)

The child fought further along in the Last Great Time War than his predecessor. Despite this, he was able to manipulate events to write out many of his actions during the War, even though it was time-locked. One of these events was presenting himself as a messianic figure to a Sontaran colony world, creating a breakaway religion worshipping his pillar TARDIS and growing goatees, so that they would battle with the main faction of Sontarans' 17th Veteran Cohort over this. (COMIC: The Judas Goatee)

Alliance with the Doctor


In the midst of the war, the child made a deal with the War Doctor to end their old ways and become allies, fighting alongside the Doctor. The (COMIC: The Organ Grinder) At some point, they sat together on a barren planet strewn with destroyed Daleks and a ruined building resembling a destroyed Capitol (though whether this was actually any version of Gallifrey is unclear); despite being time-locked, this event was unexpectedly, briefly visited by Missy and when using the Doctor's TARDIS to search the Master's timeline for an alleged way to access part of the Key to Time. (COMIC: The Master Plan)

Eventually the War Doctor was sent on a mission to find a way to stop the Cyclors, who had allied with the Daleks. (COMIC: Pull to Open) When fighting alongside the Thirteenth Vexillatio, the Doctor showed the child evidence of the Cyclors' involvement when the entire troops were psychically wiped out, leading him to agree to join him. (COMIC: The Organ Grinder) At some point, the Squire also joined the Doctor's mission to stop the Cyclors. The group fought the Daleks and Cyclors on Veestrax, where they saw a broken wall with "Exterminhate" written on it. The child told the Doctor that he hadn't written it. The Doctor then asked for the child's help with destroying the planet. (COMIC: Outrun)

In order to infiltrate the center of Golgauth, the home planet of the Overcaste to whom the Cyclors were bonded, they embarked on quests to acquire tools such as a megarhythmic anodizer from Chroleen, a mimetic whisperfield from the Terrorsmiths of Diwoon, and most importantly the Psilent Songbox of Karn. During the final preparations for their plan, the Doctor and the Master encountered the Volatix Cabal on Lujhimene, proving the rumours of their existence true. (COMIC: The Organ Grinder)

Alice's intervention
At this point, Alice Obiefune, a companion of the Eleventh Doctor in the post-Time War universe seeking answers to its events, used the future version of the Master's TARDIS and its chronal tumour to attempt to break through the time lock. To do this, the corrupted TARDIS took control of her mind (represented to her as its owner, the small boy, at her day job as a librarian), and crash-landed in the past version of Lujhimene. (COMIC: First Rule) Here she encountered the War Doctor and the small boy, but was quickly captured by the Volatix Cabal. However, the Squire rescued her and brought her back to the Doctor and the small boy she saw in her vision, whom the Doctor identified to her as being nameless, but formerly having been known as the Master. The boy reclaimed his TARDIS from "some tangled tomorrow", which could still communicate mentally to him, and connected it to the Doctor's as they headed for Golgauth.

With both TARDISes in tow, they travelled beneath the surface of Golgauth, where an Overcaste rebel base was located. The Master enjoyed the idea of using the Psilent Songbox to wipe out the Overcaste and thus the Cyclors (mainly because it would mean the Doctor would commit genocide in a manner hypocritical to his pre-war self), but the Doctor claimed to have another possible way. Upon arriving, they met a group of Overcaste, but uncovered a Volatix spy in the group and neutralised him. The spy revealed that he had called for help and moments later a Cyclor tore off the roof of the base. (COMIC: The Organ Grinder)

As the Cyclors approached, the child taunted Alice about how her beloved Doctor had committed genocide in the Time War. Due to the temporal instability, she briefly saw him as his past self. The War Doctor revealed his plan to use the Psilent Songbox to affect the Overcaste's collective unconscious such that the Cyclors would ascend. As Alice stole the Songbox in an attempt to avoid the histoyy she knew, the child used a device to summon a squadron of Gallifreyan ships to attack the Cyclors. He used this chaos of the Cyclor attack to flee, sneaking into his future self's TARDIS to escape the Time War. (COMIC: Kill God)

Fate
As the Master began to take off in his reclaimed TARDIS, he realised that it was his future self who had implanted the chronal tumour on his TARDIS console in the first place. At some point in his now aborted future, he was to operate on the brain of his TARDIS, which would result in the chronal tumour that protruded from one side of the console. (COMIC: The One) But since escaping the Time War in his TARDIS would preclude him from implanting the tumour in the first place, the resulting paradox shattered reality in "a cascade of temporal possibility." Subsequently, the chronal tumour began to react to the Psilent Songbox, creating a chronal meltdown. While the child attempted to disarm the device, it was too late, and the resulting temporal bomb left the time-ship spiralling out of control. With the child's timeline collapsing as a result of the paradox, reverted him to his previous incarnation of the War Master. The Squire would later note he had been flung "who knows when." (COMIC: Fast Asleep)

Due to the paradox, all parties present would lose their memories of the events, though the exact extent of this was not clear. (COMIC: Fast Asleep) When revisiting her timeline and seeing this incarnation alongside the War Doctor, Missy claimed to not remember which was her and which was the Doctor, implying she may have forgotten this incarnation entirely. (COMIC: The Master Plan) Alice did recall the Master being present as a small boy, and at least the fact that the Master was present in the Cyclor event surprised the Eleventh Doctor; Alice's memories of her time in the Time War seemed to soon fade from her. (COMIC: Gently Pulls the Strings) Before she had gone back to the war, the Master's TARDIS had also printed an image of the child incarnation alongside Alice which the Doctor kept secret. (COMIC: The One, First Rule)

Psychological profile
While fighting in the Last Great Time War, the once-Master ceased using his former title. As the Warrior known formerly as "the Doctor" stated, this was the nature of war. (COMIC: The Organ Grinder) However, others, such as Alice Obiefune and the Squire, still referred to him by this title. (COMIC: Gently Pulls the Strings)

The child delighted in the prospect of committing genocide, and more specifically the War Doctor's loosening morality. He saw people as resources, and that teaming up with the War Doctor was a worthwhile option during wartime. He also had a fondness for paradoxes.(COMIC: The Organ Grinder) The child wanted to put up a fight for good, but seeing the hopelessness of his efforts with the Doctor he fled from their fight with the Cyclors. He concluded that he enjoyed death and chaos only when it was fun for him. (COMIC: Kill God) Although the Eleventh Doctor may not have retained memories of this incarnation of the Master, he recognized his ploy with the Apostasy of the Goatee as the work of his old foe due to the twisted sense of humour. (COMIC: The Judas Goatee)

At the moment of his retro-regeneration, the child held contempt for the Doctor and Alice Obiefune, declaring that he would have his revenge on them. (COMIC: Fast Asleep)

Appearance and clothing
In this incarnation, the former-Master's body was that of a small child with black hair and blue eyes. (COMIC: The Organ Grinder) On Veestrax, he had short hair, and wore a white shirt with a black jacket. (COMIC: Outrun) By the time he and the Doctor had arrived on Golgauth, the boy's hair was longer, and he sported a scarlet shirt under his jacket. (COMIC: The Organ Grinder)

Behind the scenes

 * This incarnation's final moments in Fast Asleep (the last regular story to feature this "Master") where he turns into the likeness of Derek Jacobi was initially interpreted by many readers (and even this wiki) as a usual regeneration, likely due to the visual appearance of golden light seen in post-2005 stories and Alice's narration that "It is impossible to survive the energies in the heart of a paradox." However, Rob Williams, who wrote the comic, clarified that the "Child Master" was intended to be the successor of Jacobi's War Master who was reverted, rather than his predecessor or a younger version of him.
 * Although this incarnation does not use the name "the Master" and never indicates a preferred title in-universe, the recap page of the twelfth issue referred to him as "the Child Master" in a similar fashion to "War Doctor". (Though, it is worth nothing that the next issue's story Fast Asleep featured Alice dubbing the War Doctor so in-universe.)