Speculah:Kyubey created Mami and uses her to make more Magical girls
Basically when Mami and Kyubey are first introduced, Mami is guarding Kyubey 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 Kyubey, 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 Kyubey needed it was all too convenient. Which brings up the second point of how when Mami died, Kyubey 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 Kyubey. Which would explain Homura's hostile nature towards her.
- 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.
- In Ep 4, both Kyoko and Kyubey refer to Mami in a manner which implies that Mami was well-known in the magical girl community and her death is a notable event. As such, in it's current form this speculah is most likely disproven.
In Puella Magi Kazumi Magica, it is revealed that Kanna Hijiri is the product of magic/wish. She is a duplicate of Nico Kanna but with a different set of memories. Kanna is not a product of Kyubey's design but of Nico's wish. What does this imply is still unknown but it is revealed that as an artificial lifeform with a soul she is not limited to contract with Kyubey.