Intelligent, young man. Though he lacks experience. Master of Rider at the 4th Grail war.
Weaver was a 19-year old student at the Clock Tower, an institution hidden within the British Museum that teach young magi and was notorious for the power struggles between several magi factions. While intelligent and talented, Weaver was never recognized by his peers and lecturers. He thought of the idea that a mage's power could grow through effort, experience and knowledge instead of increasing magical potential through a mage's heirs. However, this idea was dismissed without a second thought by his lecturer and nobleman, Kayneth Archibald el-Melloi.
Out of anger towards Kayneth, Weaver would steal his lecturer's artifact he intended to use in summoning a Servant for the upcoming 4th Holy Grail War. Wanting to prove his genius to his lecturers, Weaver decided to compete in the Grail War and would summon Rider as his Servant.
For Fate/Zero, Weaver was originally made in mind as a foolish character before his character's gradual development towards later in the series reached a point where he could be mistaken as the title's protagonist. He was originally going to have more scenes in the novel to shape his development as a character, but these were removed to avoid further confusion over his role within the series. For his voice actor in the anime adaptation of Fate/Zero, he was voiced by Namikawa Daisuke in Japanese and Lucien Dodge in English.
Weaver has greenish-black hair and eyes. Throughout much of Fate/ Zero, he would be wearing a green formal sweater with a white dress shirt and tie beneath it with green dress pants and a pair of dress shoes.
Despite his talents and intellect, Weaver is a timid person by nature as he isn't courageous and can cower to intimidating presences such as his lecturer, Kayneth. However, he does has an inner sense of justice and courage, as he learns to get past his timid personality throughout Fate/ Zero thanks to his Servant, Iskander.
Arriving in Japan, Rider and Weaver would take residence with the Canadian couple, Glen and Martha Mackenzie, throughout the span of the Grail War, where Weaver would use his magic to make the couple believe he was their grandson. Rider's overbearing personality and fascination with modern life, paired with Weaver's timid personality, made their relationship a shaky one at first due to their differing personalities and Rider's self-centered nature. Despite this, both had a close bond together as Rider still cared for his Master's well-being and dignity, even defending him when he was getting berated by Kayneth for stealing his right to summon Rider for the Grail War. The bond between the two would strengthen throughout the course of the series, with Rider teaching Weaver of the joys of living like a man.
On the final day of the Grail War, Weaver would renounce his right as a Master to Rider by using all of his Command Spells in succession to allow for Rider's freedom, afraid of the risks to come. However, Rider is able to talk Weaver in accompanying him into the final battles of the War as his friend, not his Master. In their battle with Gilgamesh, Weaver consents to being Rider's subordinate, the latter ordering his former Master to live on as he is later defeated in the battle. Despite being afraid, Weaver stands up to Gilgamesh when the golden-armored Servant confronts him, declaring that he is a follower of Iskander and has been ordered to live on. Out of respect for his opponent, Gilgamesh would spare Weaver's life.
Surviving the Grail War, he returns to the MacKenzie residence and decides that he should stop studying for a while and experience the outside world, asking to stay with Glen and Martha.
Note: The information presented here is based on details covered in a Character Material book covering events after the Holy Grail War and with Fate/ Stay Night.
Weaver would later return to London at 29-30 years old, being a changed man thanks to his time with Rider during the 4th Holy Grail War. He would aid in the restoration of stability in the Archibald family, who were in shambles following Kayneth's assassination by Kiritsugu in the Grail War. While Weaver and Kayneth were on bad terms with one another, the former was still recognized as an unofficial apprentice to the latter. His actions led Weaver to become the tenth head of the Archibald family, given the title Lord El-Melloi II.
With this title, Weaver became a famous lecturer at Clock Tower for recognizing the hidden talent of magi and training them in it, leading his students to achieve high ranks as magi. While earning a number of nicknames and being very popular with the academy's female student body, Weaver has no interest in his fame and is actually quite irritated with it, leading him to have a constantly cranky face in the several years he had been at Clock Tower.
Weaver would become a sponsor for Rin Tohsaka in the Mage's Association, as the two later came together to completely dismantle whatever remained of the Greater Grail following the 5th Holy Grail War and put an end to the Grail Wars in Fuyuki City.
Being a talented magi, Weaver has demonstrated skills in alchemy and memory manipulation. While his status as Lord El-Melloi II gives him considerable influence within the Mage's Association, his skills as a magus are still average and he continues to improve upon them.
Inheriting Kayneth's Volumen Hydragyrum upon becoming Lord El-Melloi II, Weaver had altered its functions to make it into a humanoid automaton called Maid Golem, having properties similar to the T-1000 cyborg from the Terminator II movie and used to perform house chores. However, it suffers from a bug where it will usually go out of control and believe it is a deadly robot from the future, possibly coming from exposure to the Terminator II movie during its developmental stages.