It took me a couple of episodes to get into this series, and it's not one I'll soon forget. It has some of the most memorable - and tragic - figures in Anime. No giant robots or wizards, demons or aliens. This is the story of professional assassins, brainwashed and brutally trained into their profession. We stare with Einz and Zwei, then later add Drei. All three are high-school aged kids, and all are the best at their profession. This is realistic, sometimes heart-wrenching, usually action-packed animation. It's similar to Ghost in the Shell in it's look and feel, only set in modern times with modern weapons. Mob bosses using specially trained assassins to keep other mob families in line. We have unrequited love, brutal assassinations (some are pretty shocking), world-class animation and voice actors, and is one of the best I have seen in ages.
The ending - the real ending - is in the final credits, and it's a shocker. Is it open ended? Not really, but I suppose a third season could be produced with appropriate explanations, although these would border on the unbelievable. You'll find yourself cheering for these three assassins, and when they inevitably have to face off against each other, you're not sure who to cheer for. On the other hand, all are cold-blooded killers, so why cheer for any of them? Because you grow to like these psychopaths.
Many of the other main characters in this series are emotionally broken, cold and calculating, manipulative - you name it. When a character meets his or her end, it's often a total shock, but done masterfully.
Do yourself a favor and get this series. It's amazingly well done, from the knife fights to gun battles, hand-to-hand to - well, you name it.