One of the playable characters in Fate/EXTELLA LINK. Looks a lot like the male protagonist from Fate/Grand Order. Incidentally, Charlemagne was incorporated into FGO as a playable Saber-class Servant during the Traum Lostbelt story scenario.
True Name: Charlemagne, King of Franks and Leader of the 12 Paladins. Specifically, this is the incarnation of the fantasy stories surrounding Charlemagne, whereas Karl Der Grosse is the real Charlemagne as he was in life.