Youthief is a dumb one, he barely uses his stealth ability and when our heroes first fought him, he was easily defeated. He managed to escape from their clutches and got the money that he stole thanks to one cheap trick involving a cardboard cutout, a bag, and a tiny speaker. Yes, they were fooled by that but I can't blame them, the trick was really good. Even if he has some neat cheap tricks in his sleeve, he is still quite dumb for one simple reason; his plan for this game is to cripple the economy by stealing money.
This is where the Amu stuff comes in and how her involvement caused one security guard to get his spotlight. Unlike the previous character focused episodes wherein there is this balance between the hero and the guest star, this episode feels like it is more focused on the security guard and his sick sister. Amu felt like a mere observer in this situation and not a direct participant. To be honest, there is nothing wrong with it but it doesn't further her character unlike the previous character focused episodes before it.
As expected, the next day, they executed the plan and our heroes are now wrapped and now stuck in some room with the security guard. This is where Amu being the mere observant comes in, she interacted with the guard and his sister, but it didn't further her character arc. She is still the voice of reason of the team, and this is what she demonstrated when she made the guard realize what his sister will think if she finds out that the money for her operation was stolen money. Thanks to this, the guard releases the team and they once again fight Youthief.
The episode ends and the guard decide to return the money and continue on working hard for her sister. This is not a bad end for the episode, but to be honest, I find the ending a bit of a downer. I mean, we don't know if his sister will get an operation of even if she has the chance to continue on living after the series ended. I guess this ending is for the best, let this be a lesson to a lot of kids that life doesn't feature a lot of happy endings. All we have do to is continue on living, and that's what happened to the guard. He will continue on living and save money to save his sister's life.