Doris Ruridou is young princess who had just entered Teito Academy. She is also a Panzer, a person with special abilities and fights with a weapon called a "tool". Shortly after entering, her older sister shows up to teach her how to fight as a Panzer against other Panzers who challenge her.
D4 Princess is one of those shows that most people will overlook. Unless the superficial reading of the plot and genre tags interest you, there honestly isn't much of a draw for the average person. As someone who is very interested in all things that combine magical girl-esque transformations and mecha-esque metal combat suits, D4 Princess was something to look forward to. In that regard, the overall aesthetic looks really good.
The odd thing is the show chooses to pull a few tricks. With the short episode length, the typical thing to do as a creator would be to stuff in as much silly antics
and zany combat as possible at a frenetic pace. However, none of this happens, as combat takes a back seat to the main character, Doris, herself.
This might be why the anime is so addicting. Taking a rich and spoiled young girl (literally a princess) and stuffing her into a very luxurious academy, wouldn't usually leave too much room for pity on behalf of the main character. However, the anime decides to forego typical rich girl 'ohohoho' type nonsense and, through the constant misfortune that befalls Doris, the anime manages to create one of the cutest and sympathetic characters I've seen in a long time. Whether it's the torturous training her (probably secretly sadistic) older sister puts her through everyday or the bad luck that befalls her at the cafeteria, Doris has a really tough time. And seeing what she goes through it's impossible not to feel sorry for her, and as a result it's impossible to stop watching. It's an instance of, despite her implied status, the fact that everything goes so wrong for her and she can't seem to get anyone to understand or care, and the frustration that results, she's relateable. Sitting there, waiting for another character to finally pity her, and when it happens it makes you feel good. The way she acts and the situations she's thrust into makes her a powerful moe character.
And none of this has anything to do with what you'd think the meat of the show would be, comedic magical mecha combat.
Because of this focus, the pacing, despite the short episodes, is incredibly (surprisingly) well done. The first few episodes are literally just Doris getting used to school. This slow, but natural build, up gives the anime the feel of a typical length show. The only problem with this is the combat doesn't get as much focus, and when it does, it's underwhelming. The battles are not as zany, fun, well designed or as interestingly animated as they should be. The only thing helping it, is that you actually want Doris to win. The comedy starts off hit or miss but improves as the viewer gets used to Doris.
The ending is unfortunate, not the result, but the way it's done. The art is colorful. The ED is great.
D4 Princess has some surprising elements, with a much stronger character focus than expected. Overall, I'd say the show is worth watching just for Doris herself, and considering she's the main character there's nothing necessarily bad about that.