[Snake]

Ensui could be a shield that increases based on half the damage you take and blocks all damage taken by LV, and goes for 3 rounds. Similar to Narcus, but defensively.

Kirosh could be a way to bypass Darkness Resistance, I agree. Something like 'Also ignores enemy Darkness Resistance, but you suffer -30% Light Weakness'.

Setsuna could be a way to bypass Light Resistance. Something like 'Also ignores enemy Light Resistance, but you suffer -30% Darkness Weakness'.

All swords can only have the extra effect paired with their main effect, if main-handed. So they don't stack with each other. (Or even be future potentials for Dev to add on.)