Leah is a construct created by Hela just after Hel's battle with the Disir, after the events of Siege. Unlike the other constructs, Leah is ( obviously ) capable of independent thought and serves both as a second opinion for Hela and as a safeguard against anything happening to Hel if Hela herself should be compromised. While she lacks Hela's memories and knowledge, Leah has been informed of all the events leading to her creation and of the majority of following events as perceived in Hel. Her magic, however, is roughly the same in power as her mistress' as it wouldn't do to have a possible ruler of Hel be less powerful than the previous one.