Bonders, like their name suggests, favor bonding with a small number of Youkai. Through that bond, the Youkai and the Bonder work in harmony, bringing out the potential in each. To a Bonder, Youkai are not only tools of power, but irreplacable friends.
Potent passive skills when using Bonded Youkai.
Many abilities useful for keeping Bonded Youkai close to your enemies.
While you have less Youkai, that also means less Youkai that need to be trained.
White Spirit + Fast Offense = Youkai dealing insane damage.
Share Pain allows your Bonded Youkai to take less damage, and thus, survive better than a Grand Summoner's.
You can only have up to 3 Youkai, greatly cutting down the versatility that Summoners normally have.
On top of needing to grind for Levels, you'll also have to grind Friendship to keep your Youkai happy so they can count as Bonded Youkai.
Have no more than 3 Youkai and win a battle with at least 20 levels in Summoner. Alternatively, getting 60% Friendship with a Youkai will also unlock it.