Kill your character? I think a lot of plotting should be done beforehand. Mami can only be killed if Yuno shatters her Soul Gem.
Torture your character? Pretty much the same as above. But Mami doesn't quite feel physical pain. Kyuubey has stated the Puella Magi's bodies are more resilient to pain and injury. If they push themselves far enough, they won't be able to feel anything at all.
Stalk your character? Sure. o/ She can try.
Kidnap your character? Ditto.
Fight your character? Sure. Mami uses magic to fight though.
Anything I may have forgotten for now? Nothing comes to mind. \o/