Kayneth El-Melloi Archibald

A genius magus, famed as a prodigy ever since his days as a student. He is the youngest person ever to become a lecturer in the history of the Department of Spiritual Evocation. Skilled not only at research but also political
maneuvering, with access to a multitude of influential connections due to his family's excellent pedigree, it was expected that one day he would form the most powerful faction in the Association.
The reason he chose to participate in the Holy Grail War was not out of desperation or necessity, but simply because he wanted to add some “martial exploits” to his life story. Though he did not take the “three founding families” lightly, he did brag that “I, Lord El-Melloi will be able to overwhelm them with my wits alone.”
As for why he had so many terrifyingly dangerous mystic codes prepared despite being a researcher, well, that was just his hobby. As it turns out, he actually had talents in numerous schools of sorcery, not just spiritual evocation. In other words, he was a totally preposterous jack of all trades, just like Cao Cao or Hiraga Gennai.
Probably, he even counted the Heroic Spirit employed as his Servant as one of his numerous mystic codes. And, because he did not recognize his Servant as an individual, he was unable to understand Diarmuid's loyalty even to the bitter end. This was the root cause of his eventual self-destruction.
However, Kayneth cannot take all the credit for this failure. Diarmuid, too, was at fault. Because he would have been happy with any Master that accepted his loyalty, he failed to understand Kayneth as a person. If he had showered Kayneth with praise and admiration of his genius, Kayneth probably would have treated him more tenderly in return.
Of course, being unable to form a bond of mutual trust with his Servant was only the second worst thing that happened to Kayneth. The first, by a considerable margin, was encountering Emiya Kiritsugu. Though Kayneth definitely outclassed Kiritsugu as a magus, he couldn't hope to match him as a killer. In a battle to the death like the Holy Grail War, Kayneth's defeat was all but certain.
Thanks to his untimely death, the piles of valuable unfinished research he left back at the Clock Tower were left to rot away in neglect, and were very nearly lost. However, to everybody's surprise his least talented former student came to the rescue, and used his strange, savant-like talent of “re-interpreting and classifying theories despite having no practical ability at all” to consolidate everything into a single grimoire entitled “Lord Kayneth's Encyclopedia of Secret Arts.” Afterward, he entrusted the tome to the Archibald family, thereby ensuring their future prosperity.
By the way, in case anybody was wondering how Team Lancer could have won the Fourth Holy Grail War…
・ First, Kayneth must keep his Servant from awakening to things like glory in battle or a knight's pride. They must approach the Holy Grail War as mere dirty work.
・ Second, in order to fulfil the previous condition they must avoid contact with Saber at all costs.
・ Third, they must make securing a path of retreat from battles their top priority. Consider it a success if Lancer is able to get in a single hit with Gae Buidhe. By repeating this tactic over and over, they will be able to exhaust all of the enemy teams.
・ Finally, they must take advantage of Team Berserker. Though Berserker was the greatest threat to all the other Servants, he had poor compatibility against Lancer. Thus, if at all possible it would be in Team Lancer's best interest to allow them to stick around to the very end and eliminate all the other teams, before swooping in at the last second to take them out.

Fate/Zero material: Fate/Zero Encylopedia