[Autumn]

I think it would be more fair to the other katanas if they gained more proficiency in battle other than having solely on-kill effects, they are not at all considered much compared to the likes of Raijin, or even Hisen now, so I think it reasonably stands to amp up their potentials to have a bit more usefulness for the swordsmen who don't want to be lightning themed (rare as they may be.)

Here are my current suggestions:

Kouenjin / Crimson Lotus

Potential Skill - Crimson Lotus
Kouenjin's Potential Skill. The Kensei skill Hidden Cut's effect changes in the following ways; Damage becomes Fire magic damage. Monsters who are defeated by this effect explode, knocking away field objects and dealing Fire magic damage equal to Hidden Cut's damage in a 3 Range circle around them.

Changed to:

--Enemies who are targeted by this effect explode, knocking away field objects and dealing Fire magic damage equal to 50% of Hidden Cut's damage in a 3 range circle around them.

If an enemy is killed by this effect, inflicts certain defeat.

Yukijin

Potential Skill - Ice Lily
Yukijin's Potential Skill. The Kensei skill Hidden Cut's effect changes in the following ways; Damage becomes Ice magic damage. Monsters who are defeated by this effect turn into Ice Points and create Ice Sheets in a 2 range diamond around themself, which last for 3 rounds each (The LV for the Ice Point created is 10).

Changed to:

Enemies who are hit by this effect may be frozen for 2 rounds (with an infliction check), and create Ice Sheets in a 2 range diamond around themself, which last for 3 rounds each. (Frozen Level = 25)

If an enemy is killed by this effect, gain Ice Point Guard status for 3 rounds.

Akujin

Potential Skill - Dark Moon
Akuijin's Potential Skill. The Kensei skill Hidden Cut's effect changes in the following ways; Damage becomes Dark magic damage. Monsters who are defeated by this effect melt, becoming Dark Water tiles in a 2 Range circle for 3 rounds.

Changed to:

--Enemies who are targeted by this effect gain Dark Resistance Down. (LV = 15, can stack twice), monsters who are defeated by this effect melt, becoming Dark Water tiles in a 2 Range circle for 3 rounds.

Sanjin

Potential Skill - Mossy Grave
Sanjin's Potential Skill. The Kensei skill Hidden Cut's effect changes in the following ways; Damage becomes Acid magic damage. Monsters who are defeated by this effect melt, becoming LV 30 Poison Pool tiles in a 2 Range circle for 3 rounds.

Changed to:

--Enemies who are targeted by this effect begin to melt, gaining lvl 5 Acidity for 3 rounds, increased to 10 against heavily armored enemies. Monsters who are defeated by this effect melt completely, becoming lvl 30 Poison Pool tiles in a 2 Range circle for 3 rounds.

Either way it goes, I do think they should sit up there in the cool and viable squad alongside the aforementioned Hisen and Raijin.