Ah, okay, I see what you're trying to do. @characters has the graphics name. The following code should work, but it won't give you what you're after.
for i in 0...@characters.size
player_names += @characters[i][0] + " "
end
For what you want, you'd have to load $game_party, then iterate through $game_party.actors to get their names.
However, $game_party is way down in the list, and you might find it slow to bring up the loading screen if it has to show for several save files.