British comics writer and game journalist, known for his series ''ComicBook/{{Phonogram}}'' and several Marvel books, including ''Uncanny ComicBook/{{X-Men}}'', ''JourneyIntoMystery'', and ''YoungAvengers''. He also helped found, and for a long while wrote for [[http://www.rockpapershotgun.com/ Rock, Paper, Shotgun]], and wrote the creator-owned mini-series ''Three''. Currently, he's writing ''IronMan'' for Marvel, and the creator-owned titles ''ComicBook/{{Uber}}'', and ''The Wicked and the Divine''.

Many of his best-known works, including ''Phonogram'', ''Young Avengers'', and ''The Wicked and the Divine'', are collaborations with artist Creator/JamieMcKelvie.
!!Tropes present in his works and in real life applying to him:
* AuthorAppeal: Pop culture, especially pop music. ''ComicBook/{{Phonogram}}'' is a love song to Britpop and ''YoungAvengers'' is full of pop culture references.
* BerserkButton: Evidently, DoubleStandard and sexist lines of thinking, and general distaste towards people who treat women negatively. He tends to be calm and respectable, but he loses his snarky CloudCuckooLander exterior.
* BritishAccents: Gillen has one, unsurprisingly.
* BrokenBase: Similar to his friend and fellow writer Matt Fraction, he's gained something of this with his work on ''YoungAvengers'' and ''IronMan'' for ''MarvelNOW''. Both are either clever, fun books, or pretentious drivel, while he himself is one of the best writers out there or an overhyped hack. It appears this is partially the result of HypeBacklash, though.
* CloudCuckooLander: Comes across as this when talking to people, evidenced by his semi-joking ramblings during letter responses.
* MotorMouth: He talks so long in interviews that he often forgets the question or starts interviewing another person.
* [[ShownTheirWork Shown His Work]]:
** ''ComicBook/{{Phonogram}}''. Both the original comics and the collected editions have a ''glossary'' in the back!
** ''ComicBook/{{Uber}}'' for which Gillen wrote a 25,000-word world bible. He notes he was so deep in research that at one point his German friends have found him with map of 1945 Berlin and pile of Nazi biographies, ''World At War'' on TV, and listening to Creator/RichardWagner.
** Working on ''ComicBook/{{Three}}'' he is in close contact with a historian specializing in the period, to get it as accurate as possible.
** In ''YoungAvengers #3'', Billy at one point goes into detail about Norse Mythology, specifically the changes made to it by Marvel, such as Loki becoming Thor's adoptive brother and nemesis rather than his best friend. On Tumblr, Gillen admits that he would've had him go into more detail about his knowledge of Norse Mythology, but the scene would've dragged on.
* {{Keet}}: He's certainly a excitable fellow.
* OneOfUs: A given, being a comic book writer, but especially notable from his Tumblr account and the general presentation of ''Young Avengers'', which includes him drawing attention to fanart, and his regular references to such topics as slash-fiction and fandom.
** His comics fanboy comes out in afterwords to ''Uber #0'' in which you can ''feel'' how happy he was when GarthEnnis read his world-bible and said it has potential.
** He is also found of works of JunjiIto, calling ''Manga/{{Uzumaki}}'' his favorite horror comics.
* ProductionPosse: Several of his books (''ComicBook/{{Phonogram}}'', ''YoungAvengers'', ''The Wicked and the Divine'') are done with artist Creator/JamieMcKelvie, colorist Matt Wilson, and letterer Clayton Cowles.
* SnarkKnight: The aforementioned ''ComicBook/{{Phonogram}}'' glossary is marvelously snarky. It's entertaining enough in its own right to justify buying the comics, even if the actual comic bit sucked. (Which it doesn't.)
* SpiritualAntithesis: His creator-owned titles have a tendency to be explicit critiques of other works. In particular, ''ComicBook/{{Uber}}'' is a deconstruction of the "UsefulNotes/WorldWarII would have been '''awesome''' with [[StupidJetpackHitler superheroes]]" sub-genre, and ''Three'' is an exposť of the actual brutality and inhumanity of the culture of ancient Sparta that was idealised in a certain famous graphic novel by Creator/FrankMiller. ''The Wicked and the Divine'' is, according to WordOfGod, a deliberate self-Spiritual Antithesis to ''Phonogram'' - ''Phonogram'' was about the relationship that fans have to recorded music with the actual artist being [[DeathOfTheAuthor wholly irrelevant]], while ''The Wicked and the Divine'' is completely about the relationship between fan and artist and fans as would-be artists.