Set in a rural town, Kouta is the only son and heir to the Mikomori shrine in the small redeveloping town of Tsumagoi-chou which had been declining in popularity. He didn’t have any interest in inheriting the shrine. Instead, he focused on his studies, aiming to be a civil servant upon graduation. His widowed father talked about marrying a young girl, but then he suddenly disappeared. That girl Erina came to live with him, along with her younger sister Motoka. With the help of his childhood friend Mio and a few supporters of the shrine, they managed to continue living normal lives. One day, Erina brought home a girl Mai, who called herself his fiancée. One thing led to another, culminating with Erina ordering him to get her pregnant.

What is the most interesting magic system from fantasy, sci-fi or anime?

For me, the most interesting and complexly developed magic system would be the one Robert Jordan created for The Wheel of Time series. In his universe, some people are able to create spells, by “channeling” the magic energy of the world itself, called the One Power.
This energy can be divided into 5 types - Earth, Water, Fire, Air and Spirit - pretty common with other fantasy and mythology series.
What is different here is that, in order to create a spell, the spellcaster has to “pull” strings of one or more of the elements and weave them together in order to create a unique and sometimes very complex form.
Once the spell is done - it can be sent towards its target.
As if this is not enough complexity - the One Power is further divided into 2 halves - one that only men can access and one that only women can channel (yin-yang reference).
It is very different how men and women access the power and they also have some different abilities - ex.
only women can sense how powerful is another woman. But Jordan’s imagination does not stop here - he further more increases the complexity of this system, by adding various rules and regulations to the system - things like a way for spells to remain active, after the caster has stopped channeling, or a way for people to “sense” a spell and learn it.
Or the fact that some spells require both male and female powers together.
Or how all people have different ability to channel the One Power - and some can access more energy at once.
Or how people can form a circle and channel the power together, increasing their channeling ability greatly.
Furthermore, Jordan adds some special abilities to people - some can infuse objects with magic, some can access the dreams realm, others have a knack for healing others, etc.
Or how, anybody can try to channel more power that he or she could control and that would forever burn his ability to access the One Power. But what is the most amazing thing about the magic system in the Wheel of Time, is the combat.
Jordan’s imagination just runs wild here - the spellcasters in the series are very clever in placing spell traps and combating each other with various nasty spells.
They can do things like cut the weaves of the other spellcaster to negate a spell or to shield their opponent from being able to channel the One Power. And to throw in even some more complexity - Jordan puts spells that could reverse time, or spells that use a different power source from the One Power, and some other clever tricks. If this seems interesting to you - I strongly encourage you to read the books! There are rumors about a movie series coming out in the near future, but I have some doubts that they would be able to properly present this magic system.