Ymir Fritz was the progenitor of the Eldians. When she was a young girl, she awakened the power of the Titans and became the first Titan, the Founding Titan. After she died, her spirit was split into the Nine Titans. She is observed to be a quiet character that does not reveal her true intentions/motives.

Spoiler

She submitted to the Eldian king in life, building him his empire, brutally destroying his enemies, and bearing his children as he commanded. Ymir even went so far as to sacrifice her own life for his. Even in death, her soul remained blindly obedient to the Eldian royal family, creating Titans by hand for about 2,000 years in the unknown land surrounding the Coordinate. She has only shown signs of embracing her own free will after she encountered Eren Yeager. However, it is possible she always had some free will, as she had intentionally freed a group of pigs from their pen and let them run free as a slave. According to Eren, Ymir waited thousands of years for someone to come to her and free her from the love she felt for King Fritz. That person was Mikasa.

(Source: Attack on Titan Wiki)