Who Entered the Last Character for the Password At the End?

My guess is that it was Gaia.

Since the mobile platforms were remotely controllable by the elders it would make sense that they were also controllable by Gaia. Once Athena stopped the elders from blowing up the D-Tank she would probably have also reactivated Gaia. Duenan's Mother created Gaia and Deunan called out to her mother just before the platforms fired, so it would seem that Gaia took pity on her and the human race and helped her prevent the destruction of the D-Tank.

There's really no direct evidence for this but the chain of events seems reasonable.

I suppose the elders themselves could have changed their minds at the last second too and finished the password, but this seems inconsistent with their goal to destroy humanity, something they've been working on for years.