Since you have a boyfriend that is an engineer you can draw from his experience. See if he can design a "spin" for your puppet that attaches to the backpack.  The second image shows a dramatic arch to the spine and this would work well for you so the head and shoulders are above your own head. This would be the base to start with.  Construct a shoulder of sorts that attach to this spin that you hang the arms from. The hips and legs of your puppet could hang off the lower half of the backpack.

Instead of a cable control in the hand for the head you might want to tie the movement of the head into your own. I don't know if you have ever seen the Lion King but some of the puppets in it are built that way.  There are "strings" going from the puppeteers head to the head of the puppet so that when the puppeteer moves their head the puppets head moves.  If you think that would be to restrictive you may want to simply make the head static.  Believe me folks well still be impressed with your puppet even if the head does not move.

Ok first of all I would build his body from l200 foam so it will be super light. Rig a cable control for the mouth attached to an arm rod on one of the arms. my guess the whol rig should barely be 5 pounds or so. Of course rod control like Shawn mentioned would be quite nice to.

Welcome to P&S.  I think the first thing you should focus on is body build and getting the arms and hands to work the way you want them to.  Over all weight and the arm function is going to be the most dramatic part of your puppet as you interact at the convention.  Then you can work on the head.  there I would start with getting it to possibly turn, I like shawn's suggestion of having the puppets head follow your head movement.  After this work on mouth movement.

Keep in mind if the puppet is too much work to opperate you won't have as much fun interacting at the convention.  So keep it as simple as possible.

I don't know if you have ever seen the Lion King but some of the puppets in it are built that way.  There are "strings" going from the puppeteers head to the head of the puppet so that when the puppeteer moves their head the puppets head moves. 

I'm just adding for clarity: an example of this mechanism is on the Cheetah in the Lion King.