Oh oh I can cover this!
First off Soul Binder is able to heal, like Priest, although they have a lot less healing options than Priest so it'd mostly only be good for things like healing through Balrog's Blood Roar or the fire breathe in FD Phase 2.
Secondly they have an attack with adds a defense debuff to bosses, which is basically like giving every party member piercing damage.
Then, when awakening comes, Soul Binder gains a support tree with a further debuff (that I'm not actually sure if it works on bosses? If it does it must have a really long cooldown because it's a huge debuff.) and access to giving shields to party members that are as strong as 80% of the Soul Binder's HP. (i.e. if You have 4000 HP and the Soul Binder has 3500 HP you would essentially have 6800 HP instead, and the shield depletes first so you wouldn't be using potions until you lose 2800 HP!). They also get a secondary heal which doesn't use their orbs like the initial one does.
Their actual DPS is burst, more like Ice RB or Assassin, where they do more when their big skills are up then do less while on cooldowns.
I hope I was able to help!