I just had a random thought and i don't know if someone highlighted this already:
Jevil, Spamton as well as the Knight as "secret" bosses all had a special move where the battle square borders expanded to a full screen attack. However, Gerson did not have such an attack.
Coincidentally, out of all the 4 current secret bosses, Gerson is the only one to not use the shadow crystal.
Could it be that these special attacks represent "Freedom" by literally removing any borders, which is why Gerson does not have one since he didn't need the "freedom" offered by the shadow crystal?