Bedivere

Bedivere never learns during the king's life that Artoria is a woman. After witnessing the king's death, he initially cannot bring himself to return Excalibur to the Lady of the Lake because doing so would confirm that Artoria is truly gone, but he eventually completes the task. In the Fate/Grand Order Divine Realm of the Round Table version of events, Bedivere fails to return Excalibur and continues trying to correct that mistake.