At the first time, the Korean haven't used red pepper in their Kimchi. They only used the soy sauce, salt water, and etc. In the 1592-1598, the Japanese occupation in Korea made the red pepper was introduced to Korea. After that, they were using the red pepper in making kimchi. (Sook-he, Kimchi Traditional Korean Food)
The History of Kimchi
1. Kimchi in the Three Kingdoms Period (37 BCE-668 CE)
In the Three Kingdoms period, the Korean already knew about the fermented vegetables that were made by using the cucumbers, turnips, eggplants, scallions, ginger, water dropwort, and fernbrake preserved in salt, soybean paste, or less (Sook-he, Kimchi Traditional Korean Food).
Then, the Korean started to cultivate the varoius vegetable, and using more seasoning to preserve the vegetable. And Korean kimchi started to had many variation.
2. Kimchi of the Goryeo Dynasty (918-1392)
In the Goryeo period, the types of vegetables used as kimchi ingredients grew more varied and included radished, eggplants, cucumbers, Korean leeks, water dropwort, bamboo shoots, and scallions. The kimchi seasonings used to make kimchi are garlic, ginger, madder, and Chinese pepper.
Kimchi as we know it today originated in the Joseon Dynasty. The main kimchi ingredients were radish and eggplant. At that early time of Joseon Dynasty, radish was the most commonly grown vegetable. But in the late of Joseon Dynasty, kimchi is made from the cabbage with the Korean red pepper. The Korean red pepper has the less capsaicin content and more sugar content which is make the Korean red pepper become sweeter and less hot.
Source: Sook-he. Kim, Kimchi "Traditional Korean Food". Ewha Womans University Press