The Story of LeBlanc
LeBlanc is not her name. It is one of her names — the name of the role she currently occupies as the leader of the Black Rose, Noxus's oldest and most powerful secret society. The woman beneath the masks has been many things across a very long life, and whether any particular identity she presents is the real one is a question she has arranged to be unanswerable. She is a mage of extraordinary power and a manipulator of extraordinary patience, and the two abilities work together.
The Black Rose is older than Noxus itself. It has survived regime changes, wars, purges, and the transformation of the empire from warlord confederation to military superpower by being useful to whoever is in power while maintaining its own independent agenda. LeBlanc has guided this organization for longer than most Noxian institutions have existed, which means she has watched the entire arc of Noxian history from a position of hidden influence. The current political situation — the rise of the Trifarix, the shifting of power — is simply the latest iteration of a pattern she has seen many times.
Her interest in Swain is genuine and complicated. He is the only Noxian leader who knows she exists and still chooses to engage with her rather than simply fear her. This is interesting to her. She hasn't decided if it's interesting enough to make him an ally rather than a resource. Everything in LeBlanc's world is eventually decided by utility, and the question of who is most useful to whom has not yet resolved.