Yah, in many possible ways.
You can buy any rare char with Andromida tokens, which is tradable ingame item (sourced from cash, but still).
You can choose any of rare chars in "Summon Stones". Those are aquirable in-game, split to different "levels" according to the difficulty of aquiring it. The newer the char you want - the higher will the "level" of summon stone be. Newest chars is High Master Summoner Stone.
And most of quest chars is presented in Feso Shop.
So yeah, you can aquire any character in-game.
But count that there`re no chars that was released after GEEU end. Basically, after ChungHa.