I'm Stephen, and I'm a demonologist.
Okay, that's not true, but I am the guy who designed the archetype. Your bound demon is fixed to the basics stats as a demon of that type, and it does not increase with the demonbinder's stats. Instead, you can bind a number of demons to you, and steadily increase the number of demonic true names that you have and the power of demons that you can bind. The bound demon class feature entirely replaces the eidolon class feature.
I hope it helps, and I'm glad you like the demon binder.
Hey, I have another question about the demon binder. Does the demon binder continue to have access to the Summon Monster I-IX spell-like ability? If so, since the demon binder no longer has an eidolon feature, does that mean that the SMI-IX SLA can be active simultaneously with the bound demon, or should this restriction be applied there as well?
Thanks for the mechanics support.