Amalie Mainbach

Amalie Baumeister is the widowed sister-in-law of Wendelin von Benno Baumeister, having been married to Kurt Baumeister prior to his death.

After Kurt's death, she chose to revert to using her maiden name, Mainbach, for both herself and her children, to avoid some of the stigma associated with her ex-husband.

Synopsis
Amalie was the daughter from another Knight peerage household, and was arranged as a wife for Kurt between the households. She was 18 at the time, making her 12 years older than Wend and 6 years younger than Kurt. She provided Kurt with two children, sons Karl and Oskar.

A few months after her husband's death, her father-in-law, Artur Baumeister, arranges for her to become a sex-teacher for the soon-to-be-married Wendelin, but the relationship continues as she falls in love with him, and she ultimately becomes one of his mistresses. She eventually becomes pregnant with his child.

Abilities
Amalie has normal human mana, which means she cannot use magic. At one point in the novel, Wend discovers that sexual relations with him cause the other wives to increase in mana beyond their previous upper-limits (note: this is beyond even the upper-limit that capacity matching can allow), however he notices no such increase in her mana, causing him to surmise that one must have magical talents first to allow that effect. He is somewhat saddened that even with such a discovery she will not be able to learn magic.

Amalie was raised to be able to read, write, and do/check even simple math calculations, and can do so at a level similar to 5th son Erich. Once Erich and Wend leave the domain, she and the main-village headman Klaus are the only people at the head of the territory that can reliably do so, with Kurt's ability being particularly bad.
 * The fact that she can read/write is why, in both the novel and anime, the older brothers comment to Wend while at Erich's wedding in the capital that the letter from Artur indicating they have no money to send wedding gifts was almost certainly written by Amalie.

Kurt Baumeister
Amalie is the widowed ex-wife of Kurt. She shares two children with him.

Amalie met Kurt at the age of 18 for the first time on their wedding day. Since this was an arranged marriage between two different Knight peerages with only sporadic contact, this was not seen as unusual.

According to her own account (in a novel interlude told from her PoV), she was treated well by Kurt for the first three years, but then mostly ignored for the remaining six in favor of him having a mistress once she had provided him with heirs. She is not sure whether he never really loved her or whether he stopped loving her at some point, and feels inclined to believe the truth is somewhere in the middle because she says he acted like a proper and caring husband at first but also that the shift was quick.

At the time Kurt died, she was 27. She became deeply concerned for the future fate of her two children, at least one of which would have likely been heir to the Baumeister Knight peerage after Kurt.

Wendelin von Benno Baumeister
Amalie met Wend for the first time on the occasion of her wedding to Kurt. Over the next six years, she interacts with him cordially, and respects his ability to read and write, which is much better than even her own. When he leaves the territory at age 12, she gets permission from Artur to write occasional letters to be transmitted to Wend and/or Erich via the merchant groups that Margrave Breithilde sends to the territory 3 times a year.

After her husband's death, she is mostly concerned for the fate of her children, as the widows and children of an attempted assassin can expect to be judged harshly by society. Instead, Wend promises that they will be treated well, and that he will ensure a knight peerage is granted to them when they become adults (at this point in the story he is an Earl, and has authority to hand out the peerages that used to belong to the former Baron Ruckner, who schemed to send Kurt the letter detailing Wend's alleged demise in the dungeon of the Ancient Magic Civilization; the novel is inconsistent on whether one or both will receive knighthood).

About two months after Kurt's death, Artur and Margrave Breithilde attempt to find a woman appropriate to teach Wend about sexuality before his pending marriage to his first five wives. The idea is that nobles need to be able to produce an heir, and thus need to be experienced sexually. This is particularly notable, as for more important nobles, the wedding night will be observed by a maid or servant to ensure the wedding is properly consummated. For less-important nobles, such as Artur himself, a book detailing sex is provided instead. The role of sex-teacher is typically given to a woman who is a widow (and thus experienced but not cheating on their own husband), who is paid to keep it secret both from society and the soon-to-be wives themselves, though it is tacitly acknowledged by society and the wives are often aware but merely accept it. Because of this secrecy, the "arranged women" are only to be with the noble until the wedding, and both sides are expected to completely end the relationship at that point.

After failing to find any other appropriate candidates, Artur decides to ask Amalie if she would be willing to take the duty, expecting her to decline. However, at least in part due to her desire to see that Wend holds to his promise to confer the peerages in his control on her children, she accepts. A small, private hut is provided for their trysts. Unexpectedly, however, Wend treats her very sincerely and gingerly, even taking her by magic to the capital (using magic to hide their appearances) to buy things like sensual underwear and outfitting the hut with luxuries such as curtains, a bed nicer than she had ever seen before, and a quality bath/shower. She notes that his care and attention makes her feel "like a woman again," a feeling she had been convinced was gone for good.

Perhaps because of that, they don't break off the relationship once he marries, transforming her from his "arranged woman" to an actual mistress and clearly with them having feelings of actual love and affection for each other, though the rate of contact decreases as he has to spend appropriate time with his other wives. The wives are aware, however, and Elise as head-wife gives her tacit consent as long as he spends no more than once a week with Amalie. He continues to go on dates and outings in places like Breitburg or the capital with her, too, rather than only being a sexual rendezvous, mirroring his early treatment of Elise from the earliest days of their engagement and his current treatment of all his wives.

In time, her relationship with him eventually gains fuller approval of all his wives, rather than just the tacit acceptance of Elise, and she eventually moves in with them. Amalie now serves as the Baumeister mansion's housekeeper, similar in station to the formal maids and servants but distinct in that she is not officially one of them.