Class skills ranking

That exist more because of lore reasons.

Cu Chulain Berserker is supposed to be a full monster of madness, so EX. But the only zerc class Cu we have in this game is an Alter, modified by Medb, so the rank of the passive is far lower, C.

In terms of gameplay, is just C.

I remember in FSN visual novel Gilgamesh having the Magical Resistance skill Rank C (E), due to grail mud, not sure if this was maintained for the Gilgamesh of this game, but that's the origin of this style of skill description.

That's the original rank, but due to the Master's weakness (In this case MC who couldn't release Herc's full mad enhancement since he's a major hero of Greece and you need tons of Mana to do that) it got lowered hence the parentheses, but they still show it to show how strong Herc is.