Question: how did ymir turn into a titan?
Answer: Ymir was able to transform herself into the Jaw Titan (顎の巨人 Agito no Kyojin?), a 5-meter Titan. She gained this ability after she ate Marcel Galliard, a Warrior from Marley, in the year 845. ... Regeneration: Like other Titans, Ymir had shown the ability to automatically heal any injury and even regrow entire limbs.