Balance. Kunchen is good as he is already.
Isarnia is slow, frail, but comes with a high attack stat and a skill that deals damage to all with -44% defense to compensate her big problem of staying alive. That and the wizard class makes her a slow and frail but a hard hitter. She needs that -44% to be worth using over the 4* defense droppers.
Kunchen is slow, but:
- he has tanky stats with a great healing skill that cleanses
- innate resistance to def-
- Elemental link reduces damage to Holy weakness for himself and all other dark allies
- Manashield from cleric class avoids mana cuts that otherwise prevent his healing skill from firing
I’m not sure why you want him to get a buffed -44% defense ailment, when he already comes with so many other benefits, that Isarnia clearly doesn’t have.