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Claimed to be the true source of power behind the United Kingdom, Mr. Gentleman is revered by the British Library as the key to regaining the UK's dominance over the world. In the OVA, Gentleman appears to be the head of the British Library's Special Operations Division. His physical abilities are limited, but his intellect and knowledge are superior to the average man's. He is Joker's boss, and gives permission to Joker to perform his missions.
SPOILER WARNING: Information after this point may spoil the OAV/OVA, TV series and/or manga.
Mr. Gentleman, through plot developments mostly in R.O.D the TV, is revealed to be the main mastermind behind the entire I-jin project. His general purpose was to create a "container", in other words a host body, which had the ability to contain his great intellect, before his body completely decayed. By creating and mutating genes provided by notable people, and eventually creating the Gentleman Books, he proves himself to be a very dangerous and powerful mind. It is also revealed by Joker that his intellect is so vast, most of it over time leaked into "the ether". This leads the British Library to collect this latent knowledge via satellite from all over the world in order to complete Gentleman's transfer to Junior (the final body chosen as host).
It is revealed in the last two episodes of R.O.D. TV that Mr. Gentleman's intellect is so vast that he is capable of imagining an entire world by himself. Indeed, given the appropriate technology, Mr. Gentleman is able to recreate the world as he sees fit. We are able to get a sense of the scale of his knowledge by seeing that in the test recreation of only London, the British Library relied on the cloned minds of hundreds of luminaries as capable as H.G. Wells and Sir Arthur Conan Doyle.
Although the recreation of London is the most grandiose display of Mr. Gentleman's ability, this is not the first time he has changed history in this manner (as the TV series hints at). During the last volume of the newest manga, it is revealed that Mr. Gentleman has rewritten the world several times, and that the changes he made were all written in his personal diary. A member of the Dokusensha claims that it is otherwise known as "The Book of Truth". If this is the case, judging by The Joker's full description of "The Book of Truth", it may mean that Mr. Gentleman is in fact a deity of sorts, having played a part in all of human history and recorded everything in that book. Another possibility is that he had previously acquired the book, gaining its eternal knowledge, and perhaps extending its content. However, either of these possibilities open several questions that are not answered in the manga or anime. It may be more likely that the Dokusensha were misinformed.
Throughout the OVA when Mr. Gentleman is seen, he normally has a turtle in his lap. Turtles are a symbol of longevity, and as such, symbolic of Gentleman's wish to be preserved.
The concept for Mr. Gentleman might be a literary reference or even allusion to the character Mycroft Holmes, brother to the famous figure Sherlock Holmes created by Sir Arthur Conan Doyle. In "The Bruce-Partington Plans", Sherlock comments that Mycrofts intellect is vastly superior to his own, and that he works for the British Government in a very important position.
An actual quote from the book is: "Occasionally he is the British Government [...] the most indispensable man in the country." Also, the second man on the photograph to the right bears a striking resemblance in both clothing and facial features to the canonical depiction of Mycroft Holmes.