Uh, what's with the people calling it cliffhanger, it's a perfectly easy to understand ending, as for the bear - it could have even been just an hypotethical existance and the manga would still be great.
But if you realy want someone as much as come and point a finger for you, I guess I can give it a go - from the last dialog between brau and bear, we can pretty much guess that they are acquaintainces for one reason or another - this leads to a conclusion that is highly likely to be:
They are originally similiar model of robot - specifically the 'perfect AI'
Don't forget the name Roosweld(can't remember exact wording, but something similiar at the very least) which the bear bears(I couldn't skip making that pun!)..this could be just for convenience, but it could also mean that he is similiar to Atom and Abra, as in someone else's consciousness was implanted into him.
And the idea behind that person was probably something along the lines of eden project prosperity, while at the same time - hatred for the rest of humanity.
As for the manga itself - I loved the plot, but it was not the thing that I enjoyed the most - most entangling was the unique world it had, it's realy an out of line view on how humanity could develop. I saw a tonn of sci fi settings, but none of them were ones I could take seriously, but this one had realy suitable policies, as in I could accept such world easily. It may not be perfect or too complex, but it shines in it's own way.
Even though it's not something as grand as monster or 20th century boys - it certainly is worth looking at.
I avoided this manga for a few years, since the reviews despite being highly ranked were uninspiring, and not discribing the most important details, which would actually promore the manga further.
A 9/10 from me.