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Mondaijitachi ga Iseikai kara Kuru Sou Desu Yo?: Overall, I found this to be a very good show; however, it lacked some background over the characters, especially Izayoi. I'm guessing they're hoping for a second season to be able to explain it, and I hope they get it. It was quite an enjoyable season. I'm sad that delicious Pest had to die =[ Also, You had very little action overall in the last couple of episodes, which disappointed me. The final fight was quite well done though.

You is the weakest of the three by far, right now. Her potential is around the other two's, though.

 Spoiler

Izayoi's power is mentioned later as being...star breaker or something...class even before he gets his first real powerup (basically, his innate, base power level is on par with a circle 4-6 resident, Shiroyasha is from circle 3 AND is a Demon Lord from there, though, so she could take him without breaking a sweat as he is currently in the anime), his (main) ability is that he hits at the velocity required for an object to escape the gravitational pull of Earth and the Sun and escape the solar system (third cosmic velocity).
Asuka's power is shown in the final episode; she doesn't just have control over things, she has control over the underlying idea of things; she can bring out the absolute potential of something, which is why Kurousagi gave her the lance. The reason she isn't as useful in earlier episodes is because she was using her power incorrectly; it is meant to modify something else, not her own abilities. She can quite literally command an ordinary weapon to become extraordinary and it will; she's a support mage.
You only has one legendary creature "befriended" right now, which is why she isn't as strong as the others, but her ability allows her to befriend a limitless number of them and stack their abilities passively with her own; technically, she can become the strongest of the three, or at least on par with Izayoi at his maximum potential, if she can manage to gain the trust of a few top tier god-entities, like a creature Demon Lord, or a phoenix or dragon or something.