Good thing Serial Experiment Lain does not have a cute animal mascot or this would probably be another Lain post. All kidding aside I think my sadistic side is coming out with this choice.
Everything in life comes at a cost and Kyubey from Modoka Magica is the purest embodiment of this fact and is also adorable. Kyubey gives those who who wish for it what they wish for at the cost of being a magical girl. Does he completely explain what that means? No he does not but at the same time he never lies to the girls but perhaps it could be seen as cruel because Kyubey does not tell them what they are truely becoming.
An emotionless being Kyubey has but one mission and anyone who has watched the series knows what that is and why Kyubey does what it deems a smaller sacrifice at the end of the day. What is one life if the rest of the universe is safe after all?