Bedivere symbolizes very explicitely Saber's own humanity. For all Saber tried to become an ideal she was in fact very much a person (and that is quite literally the problem.) Bedivere is not present under the Lion King's rule because she is no longer human here and that's why he's missing. Likewise Bedivere was the sole witness of Saber's last moments because when Camelot fell she was no longer a king no longer a knight and when she finally accepted the simple mercy of dying the only thing she was was a human being.