Hi, only Korea flair around here. So the problem is that your questions are too broad to answer except in very broad strokes, because Korea has a written history exceeding 2000 years and has been a united country for around 1300 years. But I'll try to answer them as best as I can anyways.

what was Korea like before the Japanese came in?

I'm summing up 2000 years of political history (I'm not even discussing cultural history) in a few thousand characters, so I apologize for the enormous and inevitable generalizations. I can go into detail on any of these issues if you want.

The first states in the Korean peninsula emerged in the first millennium BC, over 2000 years ago. The most famous one nowadays is the kingdom of Joseon, which, by 200 BC, was most likely centered around Pyongyang. In 194 BC a Chinese immigrant named Weiman overthrew the king of Joseon and became the king himself. This new dynasty lasted until 108 BC, when the Han Chinese empire finally destroyed it and established four commanderies in the newly conquered territory. However, the commanderies proved costly to maintain. Two of the four commanderies were abolished within three decades, and the surviving commanderies seem to have been very autonomous.

Meanwhile state formation continued in other parts of the peninsula (southern and eastern Korea were laggards in state formation). By the fourth century AD there were major kingdoms in the peninsula, such as the Kingdom of Baekje in the southwest, centered on what is now Seoul, or the Kingdom of Goguryeo in the north, centered on the current China-North Korea border although it had significant territories outside the modern borders of Korea as well. In the southeast, a kingdom known as Silla was prominent, but it was not yet particularly powerful. In the fifth century AD Goguryeo reached its apogee, even capturing Baekje's capital in Seoul and forcing the Baekje kings to relocate their capital to a defensible position further south. Baekje, Silla, and other states in southern Korea joined together to halt Goguryeo expansionism.

In the sixth century AD Silla became a stronger kingdom, particularly by carrying out reforms based on Chinese lines (Baekje and Goguryeo had already done this to a great extent). Silla then expanded voraciously. It gained Seoul, expanded into the far northeast coastline, and killed a Baekje king. It also absorbed the remaining few small states in southern Korea, and by 563 there really were only three kingdoms left in Korea. But in 581 China became reunited. The new Chinese empires periodically invaded Goguryeo, the northernmost kingdom in the peninsula; eventually the Tang Chinese allied with the kingdom of Silla to bring down Baekje in 660, and then Goguryeo in 668. By 676 most of Korea was united under Silla (I'm generalizing enormously here). The Silla governed the peninsula until the ninth century, and this era is known for its artistic achievements such as Bulguksa Temple (which literally means "temple of the country of the Buddha").

Silla began to decline precipitously around the ninth century, with the rise of local strongmen. There were popular rebellions and political disorder at the center, and by 902 there were two strongmen confederations - one that called itself a revived Baekje and another one that called itself a revived Goguryeo - that openly defied Silla rule. In 918 the ruler of the latter was overthrown by one of his generals in a coup, and in 936 this general reunified Korea under his aegis. He was the founder of the new Goryeo dynasty.

The Goryeo dynasty in its earliest decades was more like a confederation than a centralized kingdom, but skillful kings in the late ninth and early tenth centuries managed to consolidate royal power (for instance by implementing royal examinations). The early Goryeo state was dominated by landed civilian aristocrats, who were also patrons of fine culture. But in 1170 there was a successful military coup against this state of affairs. After decades of confusion (and that's putting it mildly), a military dynasty assumed dictatorial power although they kept the Goryeo king around as a puppet. But the military regime still relied a great deal on the descendants of the civilian aristocracy.

In the 13th century the Mongols invaded Korea, and eventually the military rulers were overthrown and the Goryeo kings regained power as thoroughly subservient Mongol vassals. The Mongols were, however, overthrown by one of the last Goryeo kings, who also attempted reforms to aid the Goryeo dynasty, which, it is probably safe to say, had been declining ever since 1170. In the end the reforms didn't work. In 1392 another general founded a new dynasty, the Joseon.

The early Joseon aristocracy was, IMO, a clear continuation of the Goryeo aristocracy. But the new government sought to implement Neo-Confucian philosophy in Korea, leading to a large number of changes from the general rejection of Buddhism by the government to the end of double marriages. Neo-Confucian philosophy thrived in early Joseon Korea. The early Joseon was also when the Korean alphabet was invented, which most Koreans take a great deal of pride in.

The early Joseon era came to an end in the late 16th century. The 1560s was, for example, when the "era of factional politics" really began in the government. But the real watershed moment was the Japanese invasions of the 1590s, when Toyotomi Hideyoshi ordered Korea to be invaded in 1592 as a stepping stone for what he envisioned a Japanese empire that would conquer China. The Japanese invasions killed off a great proportion of Koreans, although the Joseon state eventually prevailed thanks to both Chinese help and Korean victories, particularly naval ones thanks to a brilliant naval commander named Yi Sunsin.

Joseon Korea endured for three more centuries after the Japanese failure to conquer it, although the Manchus also invaded it in the early 17th century. Late Joseon Korea saw an intensification of factional conflict until the first decade of the 19th century, which I'd love to talk about, but there were other changes such as a growing merchant class or new strains of thought, including the introduction of Catholic Christianity. In the early and mid 19th century the authority of the king hit rock bottom as in-laws dominated the court, and corruption was rampant. Eventually this led to widespread popular risings, although none of them really threatened the state as much.

In 1863 a Joseon prince, most often called the Daewongun, gained power and actually engaged in fairly systematic reforms for ten years. But the power of the Daewongun was not assured by the system - it was dependent on him being the blood father of the king - and in 1873 his son gained actual power. It was in this climate that the Japanese and the Koreans signed a Treaty in 1876, beginning the long Japanese colonial project in Korea; the Daewongun had previously rebuffed various Japanese attempts to sign treaties, because the Westernizing Japanese government did not adhere to traditional diplomatic protocol.

Were they ever a unified country with just one ruler?

Yes. Korea was united (at least de jure, although most of the time also de facto) between 676 and 892 and between 936 and 1910. This divided state of affairs is really ahistorical; Korea had been united for more than a thousand years by 1945, when the Americans entered Seoul and the Soviets entered Pyongyang. It's certainly much more ahistorical than the division of Germany.

What was their culture like?

This is too broad to really talk about, and I wouldn't even know where to begin. Do you have any particular interests? I can talk in depth about folk religion or art, festivals or rituals, oral literature, general nature of villages, etc.

Were they ever a peaceful country?

I do not think any major war was fought in Korea between 1638 and 1811, except for a minor political revolt in 1728. So yes.

Hopefully I helped. For Korean history books for beginners, you might want to check out A History of Korea: From Antiquity to the Present by Michael J. Seth or The History of Korea by Djun Kil Kim, I found both mostly accurate.

A follow-up: Was the Korean peninsula already defined by a Northern-Southern cultural split prior to WWII, or were the major axes of cultural variation not quite aligned north to south? As in, were the major cultural regions east vs west or coastal vs inland/highlands?