Q: If there are Heroic Spirits like Archer, who was summoned independent of time axis, is it possible for warriors from the future, who are loaded with modern weapons, to be summoned as Servants?
A: Although modern weapons are powerful, it is difficult for the users of modern weapons to be classified as Heroic Spirits. The advantage of modern weapons is "anyone can use them as long as they were trained", thus it is difficult for the user to become a Heroic Spirit, whose definition is "the one and only". The users of modern weapons are faceless heroes. If there are indeed heroes who are loaded with modern weapons, the users will not become Heroic Spirits, as they are merely "experts anyone can achieve through sufficient training". The actual Heroic Spirits being worshiped will be the "greatest weapon of that era". However, for the weapon to become a Heroic Spirit in this case, it must harbour a soul.
Q: アーチャーのように時間軸に関係なく召喚される英霊がいるならば、近代兵器で身を包んだ未来の戦士なんかもサーヴァントとして召喚されることがあるんでしょうか?
A: 近代兵器は強力ですが、近代兵器をメインとする者は英霊のカテゴリーには含まれづらいです。近代兵器の最たる利点は“鍛えれば誰でも使える”という事。ですが、そうであるが故に“たったひとりの存在”にはなりづらい。
それは顔のない英雄です。近代兵器に身を包んだ英雄がいたとするなら、英霊として扱われるのは“鍛えれば誰でもなれるエキスパート”たる所有者ではなく、“その時代でもっとも優れた兵器”そのものが英霊として祭りあげられるのでは、と。その場合、兵器そのものに魂が宿らなくてはならないのですが。
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Oldest known non-CG Servant we know of is Li Shunwen, and it might be because of lol Mool Cell.