His name is taken from the skull every Marowak has for a head.
Skull has stated in the third installment that he is 26. Given the timeline between stories, this means that he is 16 in the original and 21 in the sequel.
Shadows of Fear
Story before the story
Skull's past is not a happy one, seeing as he has had no happy memories before he became a Shadow Pokémon. At birth, his father attempted to kill him, and his mother died giving birth to him. He was then adopted by a family of Lapras, where he met his adopted sister—and lifelong friend—Plesiosaur the Lapras.
Some time later, Eldes—by then apparantley starting out on his training career—captured Skull and Plesiosaur, eventually teaching them and all his other Pokémon to read and mentally tell time. Likely years later, Skull was Shadowed, and some time later the story began.
Defeating the oppressors
Skull eventually rounded up a rebellion after eventually becoming allies with Phoenix, Greevil's Shadow Moltres, and Hydropunch, Gorigan's Shadow Poliwrath. Afterwards, they recruited a rebellion of Shadow Pokémon from the Pyramid, the place they all were given their Shadow powers.
Skull eventually avenged himself and other Pokémon by knocking Eldes and his Metagross into the sea surrounding Cipher's own base of operations, Citadark Isle. He then disbanded his rebellion, left for Kanto, and mated. A son was born to him, named Cubon, and the story ended.
Shadows of Fear: Return of Cipher
Back with a vengeance
The sequel to the original, Shadows of Fear: Return of Cipher, continues Skull's adventures. Taking place five years after the original, Cubon is at age five, and Cipher is once more willing to conquer the world, despite Skull defeating Eldes. Cipher took Skull prisoner, casting Cubon out into the wild (as he was deemed too weak to become a Shadow Pokémon) and imprisoning Skull in a building in Pyrite Town.
Escaping and barely beating a pair of stronger, improved Shadow Pokémon, Skull is then captured by Ash Ketchum once more. Escaping Ash once again, Skull eventually enlists the help of a Shadow Hypno to send him to Cipher's first laboratory, where they have taken Cubon (as they found him to be Half-Shadow, and therefore worth study). Skull is then captured by the Hexagon Brothers (who, even after five years, are still a pack of idiots) and taken to Ein, whom Cipher has re-hired. Skull escapes once again, rescues Cubon, and flees again.
Skull eventually realizes that for humans and Pokémon to truly live in harmony, evil humans must be destroyed. This leads him to crusading against Cipher once more, this time on his own. Escaping the clutches of Cipher numerous times, Skull is assisted in breaking into Team Rocket's base in Johto by a human named Zak Vangard. After realizing Zak as one of few humans who truly care for Pokémon (according to him), Skull begins to realize that, possibly, not all humans are bad.
Skull then travels back to Orre to defeat Cipher. Snattle and Lovrina, unlike the other Admins of Cipher, are left alive, seeing as Lovrina was merely acting under mind control and Snattle truly cared for his Pokémon. After taking down Greevil and Evice at the same time, Skull moves to Hoenn to live away from humans.
Shadows of Fear: Journey with a Brain
Five years following Skull's defeat of Cipher, the Frontier Brain Anabel asked him to help her win the Sinnoh league. Although Skull did accept, the conditions he did so under included no Pokéballs or orders for both him and Cubon.
Between the beginning and the league
Though there are bumps in the road—such as Anabel, at one point early on, handing Skull over to Pokémon Rangers with intent to purify him—Anabel and Skull are on somewhat good terms throughout the third installment. In addition to defeating all eight of the Sinnoh Gym Leaders, Skull also defeats a Team Galactic run by commander Saturn and Pokémon Hunter J, among other foes.
The end of the series
Finally, after a duel with a Garchomp owned by the champion Cynthia, Skull and Anabel win the Sinnoh league (though, in reality, Skull only wanted to see the new region of Sinnoh). Near the end of the fic, Ash, Michael and Paul all attempt to capture Skull, though all three fail. Skull leaves Sinnoh alone, Cubon having opted to stay with the Sneasel named Ebony.
As Skull himself says, he is a hero in the view of Pokémon for defeating the evil humans of Cipher. Most of humankind, however, have a more negative view on Skull: they view him as a renegade that needs to be put in its place. A reader could possibly view Skull as an anti-hero, simply a Pokémon that does good deeds though with violent methods.
At first a dead-serious, dark, human-loathing Marowak, Skull eventually grows more tolerant over the five-year period between which the first two stories take place. His personality becomes somewhat lighter as a wise-cracking side finally reveals itself, though the vicious aggressiveness seen throughout the series remains virtually unchanged. It is also revealed that he cares for the safety of other Pokémon that are not his enemies, and has a slight amount of mercy, though only towards humans truly kind to their Pokémon. Despite meeting several humans who treat Pokémon with kindness, Skull is still quite mistrustful of humans. He also has a fatherly side, seeing as Cubon, his son, is more precious to him than his own life.
Another notable personality trait is Skull's deep hatred of the color pink which, ironically, he sees quite a bit throughout the second fiction. Another of Skull's immense dislikes is his former trainer, the Cipher Admin Eldes. Michael, Ash Ketchum, Paul, and most members of Cipher have also garnered hatred from the Shadow Marowak (Paul in particular). In chapter 8 of Shadows of Fear: Journey with a Brain, it was revealed that Skull also loathes the Pokémon Jynx, and according to him, the feeling is mutual.
Continually appearing is Skull's rebellious nature. He believes that training Pokémon is a form of slavery, and constantly refers to trained Pokémon as "slaves" or "servants". Skull refuses to take orders, even from a kind human, and despises those who think he should. He is prone to violent anger, which usually triggers Reverse Mode, and may go on a short rampage focused on the source of his anger. This often causes humans to believe that Skull is insane, which he denies.
For the first two stories, Skull has despised the main protagonist of the anime series greatly, believing Ash to be not unlike any other human. During the first story, he struck Ash in the foot after the latter gave Skull an order to use Rock Slide. It was not until his journey through Sinnoh that Skull saw Ash to be a kind trainer, seeing that his words about how to treat Pokémon vastly contradicted those of Paul. At present, Skull has a mediocre opinion of Ash, but still has a desire to attack him as seen in chapter 11 of Pokemon XD^3.
Despite Eldes being a truly kind and loving trainer, Skull hates the former Cipher Admin with a passion. Having belonged to Eldes for most of his life, Skull now simply loathes his former trainer and issues death threats to him or anyone who mentions his name (as also seen in XD^3).
Lovrina, despite her position as an Admin in Cipher, is one of the few humans Skull has a somewhat kinder relationship with. Having been under mind control for most of her time as an Admin, Lovrina was freed when Skull yelled at her due to being unable to take more of her "valley girl" speaking style. She is one of the few humans to know Skull's name and call him by it.
Jack "Jackie" Walker
Skull also dislikes the Pokémon Ranger named Jack Walker. In Journey with a Brain, Jackie attempted to purify Skull, much to the latter's displeasure. Skull, however, fled the Ranger base before any such process could begin. Though Jackie let Skull go, the Marowak still despises him.
Another of the few humans Skull dislikes less than others is Anabel. Due to her telepathy, the Frontier Brain is also one of the few humans to know Skull's name and call him by it. In XD^3, Skull seems to only do things when told when Anabel is the one who tells him (albeit reluctantly). He also will not attack her and cares about her safety, contrary to most other humans Skull has met.
Anabel's husband, however, is on somewhat less friendly terms with the Shadow Marowak, particularly because of Rich's desire to prevent his Roserade and Lucario from expressing their feelings for one another. Skull has tackled Rich in anger twice during the first two chapters of the former's appearance, and states that the only thing keeping him from killing Rich is that it would break Anabel's heart. Skull has so far only referred to Rich as "Mistbloom", unlike the other humans he has encountered. Rich also knows Skull's name and refers to him as such.
Skull, like all Marowak, stands just under four feet tall. In fact, he resembles a normal Marowak in just about every way except for two things. The first of which is the shadowy aura radiated by all Shadow Pokémon. The second is his eye coloration: unlike the traditional Marowak red, his eyes are blue. The reason for Skull's unnatural eye color is not yet clear, though it is believed to be simply a birth defect.
Being a Shadow Pokémon, Skull is equipped with two Shadow-type attacks: Shadow End, the most commonly used throughout the series, and Shadow Panic, which is somewhat rarely used. Shadow End is extremely effective against all Pokémon Skull is put up against, though the price is a painful amount of recoil.
Skull also seems to have the mental ability to control the motions of rocks when he uses Rock Slide, sometimes surfing on rocks he controls. His other pure move, Swords Dance, is used somewhat rarely, and only used as a defensive tactic against moves such as Aerial Ace. His ability, Lightningrod, causes electric attacks to be drawn towards him in battle and, being a Ground-type, is immune to it.
Skull's main weaknesses are those of all Marowak: Ice-type, Grass-type, and Water-type attacks. Skull also seems to lack the ability to swim, due to his weakness to water, and prefers to avoid it.
It was revealed in chapter 7 of "Shadows of Fear: Journey with a Brain" that Skull inherited Rock Slide and Swords Dance from his genetic father, Bonefarious.
Appearances in other fictions
Skull has appeared only once outside of Shadows of Fear, in the fiction Watchers of Raikou, written by Gazmof of the Serebiiforums. He appeared halfway through chapter 19 and through a good portion of chapter 20.
Skull has also appeared in Pokemon XD^3: The Waves of Truth, speaking fluent human. In addition to meeting Rich, Nando, and others, Skull has developed a rivalry with a Blaziken named Torkie. It is shown that he supports the idea of Rich's Lucario and Roserade showing their true feelings for one another. Skull first appeared in chapter 11 and has so far remained in the story, providing a form of comic relief.
- Skull was first scheduled to be an average shiny Marowak with similar fighting skills. After Morpher acquired Pokémon XD, he was changed to become a Shadow Pokémon.