You may get—as in unlock or achieve—every single character on your second playthrough. However, you cannot physically and permanently have some of those characters in your party until your third playthrough.
To clarify, there are two quest lines in your second playthrough which, upon successful completion, will unlock Roka, Evelyn, and Lyphia, and although you 'unlocked' them, they can only be a part of your party after selecting the "Inherit Characters" option on your NG+ options once you finish your second playthrough and start your third.
viewtopic.php?f=11&t=102Scrolling down, looking at "Character join conditions", under "Gold Alchemy", you can see that Roka, Evelyn, and Lyphia all say at the very end "begin new game".