Reddit mentions of The Dog Shogun: The Personality and Policies of Tokugawa Tsunayoshi

Very common between rich men and poor (or poorer) women around the world.

Cixi was born to an unimportant father. She became a concubine because she was hot. She became the second woman in China (only to the empress) by giving birth to a son. After the emperor died, she brutally seized power and basically ran the Empire for over 40 years. So yeah, rags to riches.

The 5th Shogun of Japan, Tokugawa Tsunayoshi was born from a concubine who was the daughter of a greengrocer. What made him different from his brothers is that he was raised by her, which gave him a very different perspective on the whole samurai class than most of the Shoguns. She also had a much closer relationship with her son, as it is presumed that his father was trying to make Tsunayoshi "soft" in order to not present a threat to his older brother (the 4th Shogun). After he became Shogun, quite by chance, she used her closeness with him to achieve real power behind the scenes, though she didn't possess supreme executive power. So, rags to riches.

In France, it was considered gauche by the royalty to take a commoner as a mistress, but there were a lot of really poor noblewomen from the sticks that were certainly eligible. And even then, the prohibition wasn't really all that hard to get around. Madame de Pompadour was given a title to this end. Madame du Barry was born poor, and became a courtesan when she grew up, where she caught the eye of the king. She had a noble lineage forged by her pimp. She was given a title by marrying a count, which made her eligible to be the king's official mistress. (What? Yep - it was less scandalous for a married woman to be a mistress in France than an unmarried one.)

Honestly, this question is really just asking "hey, could you retell the main narrative of Japanese history for 1000 years?" It's kind of like asking "What's the relationship between the Pope and European Kings and Queens?" and it should probably be in popular questions. I mean honestly just read any textbook, since this relationship is the central political question at any point from 1200 onwards. It's not in popular questions and this would make a crappy /r/askhistorians post though, so I'll go ahead and link-filled summarize:

It depends on what time period you're talking about. Since you explicitly asked for shogun/emperor, I'll start with the first shogunate. Established in Kamakura (symbolically, far away in the traditional lands of the Minamoto clan instead of in Kyoto where the Emperor was) in 1192 as a result of the Gempei war, it stripped the emperor of most of his temporal power. Even at that point, though, the operative power was not in the Emperor himself, but rather his courtiers (see the fujiwara clan), as the Emperor himself spent most his time fulfilling the many Shinto-Daoist rituals that were cosmologically needed to keep the realm in working order (a lot of waving stuff around, purifying stuff, burning stuff, etc.) The war was really between to rival warrior families who were desperately trying to marry into the courtiers and eventually the Imperial family itself. One won, the other lost, and the winner set up an alternate power structure. From here on, power fluctuated between a few sources. At some points, Emperors would 'retire' to become monks, leave their sons to do the ritual crap, while they exercised some measure of power
from the monastery (no small irony there). In this period, around 1340, after another short war/power struggle, the Ashikaga family deposed the Kamakura shogunate and set up their own shogunate within Kyoto itself. Depending on who was shogun and who was cloistered, real power fluctuated. Sometimes even abbots of powerful temples would get in the mix. In terms of actual family ties, all 3 groups were closely linked and regularly intermarried. For a really good monograph on this interesting period, I'd see Gates of Power by Adolphson.

By the late 1400s, though, that system was breaking down altogether. Local Samurai basically acted on their own perogatives on their own land. At this point, the Sengoku Jidai (age of country at war), there are people claiming this and that in terms of rulership, and all of it is meaningless. The only thing that mattered was military strength and personal loyalty, which could be broken at the drop of a hat if the benefits were seen to outweigh the consequence. For this period, the history shelf is littered with colorful picture books about the HONORABLE SAMURAI WARRIOR and all kinds of nonsense (it is also when Shogun: Total War (and its re-make) is set). One trustworthy monograph on the ending throes, that I'd say also captures the essence of the period, would be Japonius Tyrannus by Jeroen Lamers.

The Tokugawa Shogunate, set up by the eventual victor in 1600 onwards, sought to solidify sole control. During the wars, the great temples had been almost completely obliterated, so they were out. The Shogunate removed the other threat to their power, the Imperial Court, by taking over administration and funding of the Imperial Household (and thus removing the powerful courtiers that traditionally surrounded the Emperor together). In order to leave the Emperor to his important ritual business, they very kindly removed from his household the burden of managing any land--making them completely dependent and unable to cultivate their own powerbase. While the Emperor was still seen as the ultimate source of both political and cultural legitimacy, temporal power was seen to have been devolved entirely to the Tokugawa family (who did still regularly marry daughters off to Emperors). With the Royal Baby in our thoughts, I'd say it's similar to how the UK runs now; the Emperor is around, popular, and beloved, but not even a figurehead in terms of running things. A good window into how things ran in the middle of the period would be The Dog Shogun by Beatrice Bodart-Bailey.

This is, until the "Opening of Japan" leads to everyone going nuts. For the Boshin war and what follows, I did write a post about that here just a few days ago. One thing I didn't link to in that is a book about all the neat intellectual history, which really touches on your question in terms of how intellectuals built up a sense of legitimacy for the Imperial Restoration and how that leads up to the revolution, so I'll link it here: Before the Nation by Susan Burns.