The "Familiar of Zero" features several main characters, but the predominant two are Louise, a comely young female in a strange world where magic prevails, and Saito, a boy who is magically summoned there from Earth by Louise's rite to obtain herself a "familiar." Familiars are creatures teleported into this society from wherever by all the magical (noble) people of this society when they come of age, to help their masters with their magic and battles.
The only overriding story arc throughout all 4 series is the stormy, and ofttimes comical, relationship between Louise and Saito. She is a childish, immature, jealous, and short-tempered noble who has a difficult time handling her more laidback and relatively Earthly normal boy familiar, Saito. It does get a little old to have her rage against him constantly, even as she falls in love with him, but the creators maintain that as a comic element throughout. The other nobles have summoned strange creatures like flying dragons and oversized moles and other things that don't fall into any category at all; Louise summoned another human, for which she is ridiculed, for he is one not even from her world. She also has difficulty employing her magic, which is why she is called "Louise the Zero." The only thing she can reliably do is cause explosions, which are often inappropriate while she is trying to do something else, all of which doesn't help her reputation; but it is occasionally funny.
Louise and Saito, along with their other bevy of friends, encounter a series of problems for their kingdoms throughout the 4 series, and the magical people must employ all their magic to do the battles and resolve the issues. There are monsters and bad guys and a few poignant situations which the two main characters have to confront with all of their friends, but always it is Louise, who doesn't want to admit that she's fallen in love with her familiar, that rages at and explodes her love interest, Saito, when he even looks at some of the other busty women around them. Louise herself, although quite attractive, has a slimmer build, which constantly irritates her in the face of her more zaftig girlfriends and associates, particularly when Saito looks at them and/or they are sweet on him.
Beyond that, all the noble magicians must confront these difficult situations for their kings and queens to save them, the kingdom, and sometimes the planet, from destruction by maniacal evildoers. Essentially created as a comedy, the "Familiar ..." series has some tender moments and some creditable story arcs within it. Graphics, animation, and music are routine, with nothing special about them, but they are adequate, and don't get in the way of the stories. There are some shapely, scantily clad females provided for viewers, but nothing that comes up to what you might call "fan service"; it probably could have used some to provide a little more visual interest, since there were many opportunities for it, but the creators kept it more youthfully oriented and didn't do that. Altogether, "The Familiar of Zero" is a lighthearted series with some interesting story arcs, comedy, and battle scenes, which should provide ample entertainment for those who watch it.