Integra is played by London-born Victoria Harwood.
Walter was played by Englishman Ralph Lister (from then-British Hong Kong) in the TV series and the first eight episodes of Ultimate, but in Episode 9 by American Liam O'Brien.
Semi-averted with Seras (played by Washington-born but London-raised K.T. Gray, who normally speaks with an American accent as an adult).
Most of the Round Table Conference are played by American actors, particularly in Ultimate.
Another aversion is Anderson, who is voiced by Scottish (Dundee) actor Steven Brand. Word of God says that Anderson isn't supposed to be any nationality, but at least his accent's authentic. And awesome.
Taliesin: "People keep telling me that Anderson has a terrible Irish accent. Well of course it is, he doesn't have an Irish accent!"
Fake Italian in the case of Enrico Maxwell, who is played by Englishman JB Blanc (who coincidentally, used to live with Steven Brand).
Girlycard, Alucard's young girl form. Also known as Lolicard in the Japanese fandom.
Alucard's form as Vlad Dracula III goes by either "Vladcard" or "Beardycard".
Anderplant for Anderson's monstrous form after he stabs himself with Helena's Nail.
Since the Major is never actually named, several names for him have popped up, Montana Max being the most popular.
Taliesin Jaffe, the director for Hellsing and Ultimate's dubs, calls the Major this since he's a fan of the manga.
Hans Günsche and Avondale Napyeer, fan-names for Captain and Doc, respectively.
Montana Max and Hans Günsche come from the prototypes of the characters in older works. On the back of Volume 5, you can see a small drawing of some papers with Major's face on it with some reading "Montana Max".
Incognito has been called "Vagina Head" because of the piercing and red zero on his forehead.
Follow the Leader: Several incidents in the anime are referenced in later chapters of the manga, including the Jackal's destruction, Alucard "dying", Alucard releasing to his highest for at Integra's behest, England reduced to burning rubble, impaling, Walter's "brainwashing", and the revelation that Alucard is Vlad (III) Dracula. Even if some were obvious. Also, apparently Hirano thought Alucard cocking his gun with his teeth was so cool that he decided to draw it in one of the later chapters, and also took inspiration from Incognito to create the character of Zorin Blitz.
Missing Trailer Scene: Most of the anime's early trailer's scenes are in it (or at least in the show's opening), but not that of the Major making his trademark applause gesture. He did not appear in the anime at all, as he was probably outlined by Hirano but not developed enough for the anime to build an arc around him as they Overtook the Manga.
No Export for You: For some unknown reason, and despite the popularity of the first series there, the Ultimate OVA series never reached Spain. Even when distributor company Selecta Visión announced in 2016 a surprise related to the franchise, it was revealed to be just a special edition of the Gonzo series, leaving everybody disappointed.
Dawn has never seen a release in the U.S., either subbed or dubbed.
The Other Darrin: In the English dub, Zorin Blitz was the only major character to receive a change in voice actors. Helena Taylor voiced her in OVA 4, but Taylor had to go back to England during the hiatus of the dubbing, so Rachel Robinson filled in for her for OVAs 5-7.
Young Integra was voiced by Tricia Dickson in the TV series and OVA 1 but was replaced by Laura Bailey for her appearance in OVA 5. Jaffe justified it as the character being a little older in the later flashback.
Chris Packman from the TV anime was voiced by Arthur Russell in episode 9 but was replaced by Gildart Jackson for episodes 11 and 12.
Tricia Dickson, previously mentioned above, replaced Ananda Banc from the TV series as Jessica in Ultimate. In a similar vein, the British Michael Marr was replaced with American Doug Stone as the Round Table leader Hugh Irons.
In the original anime, during the next episode previews, the Spirit of Hallconnen was voiced by ADR director Taliesin Jaffe as an in-joke based on his encounters with Kouta Hirano. When Jaffe tried to do it again for Hallconnen's appearance in Ultimate, the higher-ups deemed it "too offensive" and demanded to cast a "proper" actor. Peter Beckman was cast in the role soon after.
In Japanese, the biggest recast was Norio Wakamoto as Father Anderson, replacing Nachi Nozawa from the TV anime. This was due to Nozawa being diagnosed with lung cancer and thus was unable to reprise. Said cancer sadly claimed his life in 2010. Wakamoto was already cast in the TV series as Richard Hellsing and was replaced in that role in Ultimate by Yosuke Akimoto.
Overtook the Manga: The reason why the anime series looks so different from the OVA past some point? They caught up with manga extremely early on (as in, before the Big Bad was introduced). This was made worse by the fact that (1) Hellsing was a monthly series, and (2) Hirano is famously lazy, regularly turning in chapters only 10 pages long in a magazine where the average is 25-30. Hirano was extremely unhappy with the anime (though he still liked some things from it), and further adaptations of the manga were postponed for years until the OVA moved forward. Still, the Major (the manga/OVA's Big Bad) must have been outlined by Hirano, considering the fact that he appeared in an early trailer for the anime. His arc was replaced by Incognito's in the anime.
On the less well-known side of things, JB Blanc, Victoria Harwood and Steven Brand — the dub voice actors for Maxwell, Integra, and Anderson respectively — have known each other since the 80s. According to the DVD Commentary for OVA 3, during the casting for the original anime series, Steven Brand brought Victoria in, and Victoria brought JB.
Role Reprise: Most of the cast of the original anime series returned to their respective roles for the OVA series, in either Japanese or English (for the characters that were not unique to the anime, that is).
Schedule Slip: Ten years to finish a 10 volume manga, Kohta Hirano is (in)famously known for being quite a lazy mangaka when editors are not keeping a constant watch on him, near the end of the series it seemed Hirano would release a new chapter whenever he felt like it, he was often taking breaks from working in a monthly magazine and also some of the near end chapters are just 9 to 12 pages long!
The OVAs also followed suit in what it seems to be a staple in the franchise. The first three episodes had just a few months between each other. From the 4th and on the delays would just get longer and longer as Satelight was having a hard to time to keep producing the episodes all by themselves, after a year and a half without releasing anything Satelight passed the baton to Madhouse and finally looked like they would keep up the pace, and they did... only up to the 7th episode, another year and few months (and many rumors that Madhouse just dropped the series) is what it took for the 8th episode to get a release date; the series finally finished with the release of the 10th episode in late December 2012, nearly seven years after the series began. And the official US releases were worse, with the original license holder going bankrupt, the gap between releases was 4 years, with two Blu-Ray/DVD sets compiling the eight OVAs so far finally being released in October and November 2012 after the release of the DVD for Episode 4 in September 2008.
Hirano topped himself with the prequel, Hellsing: The Dawn, after 6 chapters he simply stopped in 2007. The Dawn didn't even have enough chapters to publish a single volume, so the series remains just a collection of 6 separate publications in 6 different Young King Ours+ issues.
Screwed by the Network: Only the first 8 episodes of Hellsing Ultimate aired on Toonami, as they only had the broadcasting rights to those episodes. This is due to an unfortunate case of miscommunication on either the Funimation staff or the Adult Swim staff, as the production crew behind Toonami were under the impression that they had the broadcasting rights to the last two episodes as well. Fortunately, Toonami aired the last two episodes as part of their December 2014 Month of Movies event.
Hellsing: The Dawn. Hirano felt like stopping out of nowhere in 2007, and yet Young King Comics did not declare the series as finished nor canceled.
The Animax English dub of Hellsing Ultimate. Animax Asia aired the series with their own English dub which has no swearing and foul language like the American dub. So far, only four episodes were dubbed and aired by Animax Asia and it's unclear if the rest of the series will be dubbed.
The TV series was produced by GONZO. For Ultimate, the first four episodes were produced by Satelight, before switching to Madhouse for the fifth through seventh episodes and Graphinica for the final three episodes.