Question: what's it like inside Jag? Since he's not human, is he warm inside, or is it cold? Does he have a heartbeat in there, or something similar?

*added to question queue*

To Jimmy: If you could buy one thing/product to best confuse Jag about human life/culture (and to best amuse you/us), what would it be?

Hmmm, tough one! Doubt there is much about human culture that could shock Jag. More likely Jag would find ways to confuse Jimmy, lol.

To jag: what its feel to be inside jimmy body? Well mostly in his stomach because jimmy cant shape shifting right?

To jimmy: what its feel to be inside jag body? Mostly in jag human body form

To Jimmy: how do seals and psychic powers /feel/ like when wielding them?

To Jag: if you can shapeshift, why do you have this form?

I'll answer the question for Jag directly: The reason for that is actually plot related! And mildly spoilery~

I'll only say that Jag can't control his human form, only his demon form. :3

Mhmmm~

I'm rather curious now, have there been any recorded incidents of demons recognizing superior strength of agent (or Jag) and following the agent instead of fighting them, on the field?
(And not like the "shadow the target, gather the pack, then jump them" type of follow.)

I'll answer that one directly too and say, that's actually a pretty common way to acquire pact demons for the Agency. A good chunk of them get taken over "second hand" from illegal summoners, cults, etc. Some are recruited directly in the field, depending on how smart the demon is, they will surrender or defect even in the middle of the fight.

Demons owe their pact masters little loyalty, if a better prospect comes along, they are likely to abandon you. A pact is undone when the pact master dies, with no consequence for the demon. Unless the pact master spends his final breath to kill the demon along with him. :V

So the best way to keep a demon loyal is...treat them like a good guard dog? Reward them and treat them well?

Depend on the individual demon, some react better to rewards, others respond to threats.