Firstly, its to my understanding that summoning the bonding Youkai can make or break a fight base on your build. Although the class can reward you for just install the bonded Youkai, summoning it out on the battlefield on the other-hand can hindering you severely if the enemy knows what they're doing. In a sense, you're better off using the other two youkais the class offers (Or just switching to GS)
It's more tied into the fact that without share pain active, the youkai can die within one to two rounds and WITH share pain active, it only increase this by one-two more rounds but can inflict large amount of damage onto the player. (Unless they bonded with certain Youkais)
My suggestion to this; Bonder class exclusive - A passive made to help with the creature's survivability so it can actually fight along side the player without it only causing problems.
Option the Bonded Youkai gets before they're allow to put points into the passive - Pick between Defensive Type or Evasion Type (Can't be changed unless the player LEs or resets via fruit)
[Insert Passive Name here]: After Selecting the type wished for the bonded Youkai, the Youkai gains these bonus whenever they're on the battlefield and standing in a certain range (3-4 squares close to the player).
Defensive - Whenever the Youkai is within range, it gains 25% of the user's Phy/Magic reduction (No battle bonus/technique/buff applies to this reduction). This effects doubles if the player only has one bonded Youkai. (Of course the soft cap/deplete should still be in consideration to prevent certain Youkai from reaching very high levels of tankie)
Evasion - Whenever the Youkai is within range, it gains 25% of the user's Hit/Evade (No battle bonus/technique/buff/Weapon applies to this bonus). This effects doubles if the player only has one bonded Youkai.