At the end of Twilight Princess, Midna broke the mirror of twilight while saying "Link... I... See you later".
As only the true ruler of twilight can break the mirror, Midna is clearly one. But why would she break the mirror as it's the only portal that connects the two worlds?
It's pretty clear that Midna destroys the Mirror of Twilight to separate the Twilight Realm from Hyrule, since she says, "... Light and shadow can't mix, as we all know. But... Never forget that there's another world bound to this one." Referring to the events that gave progressed thus far in the game.
The dialogue of before she breaks the mirror goes: "Thank you... Well, the princess [referring to Zelda] spoke truly — as long as that mirror's around, we could meet again..." After this she tearfully says "Link... I... See you later..." and then shatters the Mirror of Twilight with one of her tears.
Since she explicitly says that mirror and not a mirror it would imply that this is the only mirror connecting the two worlds.
Most likely to spare the world of light from potentially being attacked and taken over again at some point in the future.
