(The) Aegis is a term that refers simulatenously to:
Due to both of these characters being semi-clones that are paired together by a swapping mechanic, players will often collectively refer to them, such as in tier list or matchup discussion. "Aegis" comes from the name of their sword in Xenoblade Chronicles 2. Players also sometimes use Pythra instead, a portmanteau of their names, to refer to them collectively.