"Kono Subarashii Sekai ni Shukufuku wo!" (bless this wonderful world!) volume 10 by Akatsuki Natsume. In this volume, readers discover that Iris is the strongest character in this series.
Back in volume 6 when Iris first appeared, she was merely a princess who yearns for adventure. In this volume, we found that she is in fact a very capable warrior, wielding a sword named something-calibur that defeated most monsters in one strike. Given her skills, it seems like Kazuma's party is mostly redundant, and that the reason why the demon lord hasn't been defeated yet might be because no one really tried.
It's probably no coincidence that a certain something-calibur is also the name of a casino in Las Vegas. The main event for this volume is Iris's diplomatic mission to secure financial aid from a country whose primary industry is running casinos. This is where Kazuma's high level of luck and less than honorable personality really shines -- when the diplomatic talks broke down, Kazuma went on to extract money through gambling instead. It's satisfying to see players win against the house in most works involving casinos, but in Konosuba you would almost feel sorry for the casinos. Sometimes it's not obvious who are the baddies in this series, and that's what makes it so great. The truth about the demon lord oppression in this world might just come down to all the people improperly resurrected by Aqua, plus a bit of bad marketing.
Looking forward to more misadventures from the other little sister character in the next volume.
Previous (2016-12-14): "Imouto sae Ireba Ii" volume 5 by Hirasaka Yomi
Next (2017-02-03): Rainbow