[manifla]

I will try to explain some spoilers and details about the series from volumes 3 to 9
First off, the girl that Iska saved in the very beginning

Spoiler for Identity:

Is Alice's younger sister, Sisbell. She is a Pureblood member of the royal family, and she has zero combat strength, as does their eldest sister's natural power, Elletear who is seen on the cover of volume 6.
The reason why Sisbell was caught by the Empire, within the Empire, is in part because of a giant conspiracy going on within the Nebulis Royal Palace that threatens to destroy the royal family and system of governance.
Also in part because Sisbell wanted to hunt the truth about what happened 100 years ago.
And also because Sisbell didn't know who to trust anymore in her nation minus her butler, and still doesn't by the time she meets Iska again. Yes, she is in love with Iska but absolutely wants him to be her subordinate and is much much more determined than Alice is about this.

The royal family of Nebulis

Spoiler for Royalty:

Consists of three related families who are all descendants of the Great Witch Nebulis through her twin sister. Their relationships are on the surface very close on the surface, and absolutely horrible in every single way in reality. This is how things get so absolutely bad in volumes 6 and 7, and are still very bad in 8 and 9. It's a hellish mixture of conflicting ideology, greed, power, and hatred for both the Empire and the structure of the royal palace of Nebulis (yes, both of them)
Alice and Sisbell and Elletear are members of the Lou family, Mask Om and Kissing are members of the Zoa family, and the third family (not seen in the manga yet) are the Hydra. Lou represents the stars, Zoa represents the Moon, and Hydra is the Sun. Nebulis has many very powerful people who can make a huge difference on the battlefield - although the Saint Apostles we have seen are also extremely powerful and are capable of standing up to them through a mixture of insane body conditioning, tactics, technology and being combat madmen.

The Empire

Spoiler for Oh boy:

Is another source of conspiracy in this series. The Emperor is the supreme power in the country, but rarely (if ever) exercises it. In theory the Imperial Parliament is what runs the country but Iska and Jhin's master is right - be careful of the 8 Grand Apostles (not the Saint Apostles)!
They are the ones effectively running the country, and they are constantly at odds with the Emperor. There is a secret conflict going on behind the scenes between the Grand Apostles and the Emperor over where the country, and the war, and the world is heading. However, the 11 Saint Apostles are the direct subordinates of the Emperor. The Grand Apostles can make requests, and can ask the Saint Apostles for their assistance (and find ways to pressure vulnerable people close to the Saints), but they can NOT order any of the Saints. Even in Iska's case, they could only make a deal with him and not put him under any kind of time restraint (directly)
TL;DR the Grand Apostles want total war, but also are after the kind of absolute power that the Great Witch Nebulis AND the Emperor have, and have their own links to the conspiracy within the Royal Palace of Nebulis.
The Emperor does not want the war but has his own big problems. He is not human in much the same way that Nebulis is not.
Great Witch Nebulis will kill her own descendants in order to kill one more Imperial soldier. Things are pretty screwed and are only about to get worse.

Iska and Alice's relationship

Spoiler for Oh boy:

Is still a source of confusion for them both and since they are both dummies that understand nothing about love are constantly doing all kinds of irrational things about each other. Alice accidentally gets a image of naked Iska during 8 (which she keeps under her pillow) which is found by her mother in 9 and scolded for shamefully looking at "a strong muscular but innocent teenager in his nakedness!" which Alice tries to explain away as "I look at it every night to figure out everything about an enemy!" which is in the backdrop of her mother telling her that she should know the feel of a older (yes older) man instead of preying on the young with metaphors about flowers and all kinds of boomer talk

Spoilers for volumes

Spoiler for Volume 3:

The poisoning and kidnapping of Iska, but with much more detail on how Unit 907 infiltrates the enemy country, and is nearly at ground zero of Sallinga's escape instead of being near the hotel. There is a hair raising escape for the Unit from the prison, and Iska ends up fighting Sallinga on the condition that there will be no (zero) pursuit of the Imperial forces that have gotten into the country, while Alice is being attacked by one of the Saint Apostles and being kept from intervening against Sallinga, putting out the fire once and for all, or stopping the prison riot. The prison riot is serious business here, there are a lot of powerful criminals being kept beside Sallinga.
Sallinga is defeated and put back into prison, Rin and Alice get the credit, Iska and 907 and the Apostle all leave the country. Except that Sallinga has actually escaped and only a clone is in prison. And in the wilderness, the Emperor comes to him