Eren knew Sara was a fierce woman deep down. She had the mental aptitude to be a ranker.
It was just that due to her mortal status, she saw herself as among the most vulnerable bunch of people in a world where magic was normal. That's why she kept things to herself. Acting mighty without the necessary might was foolishness in her eyes.
In Eren's past timeline, Sara had to suffer from Reva's demonic runes acting on her body periodically. Her status as a blind person also made her feel inferior even among mortals.
This was the reason she became proficient in linguistic skills. Having a good vocabulary was a coping mechanism for her. Helpless people had their own ways of expressing their emotions and fierceness after all.
