Speculah:Kyubey created Mami and uses her to make more Magical girls
Basically when Mami and Kyuubey are first introduced, Mami is guarding Kyuubey from Homura in the sense that she consistently is around her, or in some way, shape, or form is around her constantly. Evidently there are many magical girls created through wishes granted by Kyuubey, yet they are no where to be found, we are to assume that Mami and Homura aren't the only magical girls in existence right now, and as such Mami being around right when Kyuubey needed it was all too convenient. Which brings up the second point of how when Mami died, Kyuubey immediately beckoned the girls to contract with him, without so much as batting an eye(like he does anything else), which brings forth the idea that Mami, or a variant of her, is used each time in the process of creating new magical girls. It first gives potential magical girls someone to look up too, relate with, and teach them the dangers of what it really meant to be a magical girl, while still making it something that would be appealing in the long run. Eventually reaching the traumatic death of "Mami" in any of her forms, forcing the hand of the girls who may be wavering to quickly become Magical girls to save their own lives, or influence them to the point of eventually becoming magical girls in the near future.
The secondary idea going off of this, is that Mami's wish, whatever it may be, forces her to relive this sort of situation with all new magical girls in order to get them to contract with Kyuubey. Which would explain Homura's hostile nature towards her.
Discrepancies-Mami show's emotion towards Madoka shortly before her death, but this could easily be counter by saying that it is a ploy to better the ties between the two before the traumatic loss forcing Madoka's hand even further.