Regarding your question, the MC's personality can change throughout the entirety of the story, you're never locked into only one set of personality traits. Though of course if you have an overwhelming amount of points leaning towards 'stoic', it might take quite a few decisions that go against that before it leans more towards 'emotional'.
I'll keep this guide as brief and spoiler free as possible. There are four possible mentors:
Lady Westwale: She'll teach you how to use a spear, as well as water divination. To impress her, you should 1) observe and remain silent, 2) express that your ideal knight is calm, 3) work together, and 4) remain steadfast.
Sir Asperame: He'll teach you how to use a rapier, as well as fire magic. To impress him, you should 1) volunteer to go first, 2) express that your ideal knight is passionate, 3) work together, and 4) forget dodging.
Chev Minne: They'll teach you how to use a sword and shield, as well as healing magic. To impress them, you should 1) observe and remain silent, 2) express that your ideal knight is tough, 3) work alone, and 4) remain steadfast.
Sir Stahlvart: He'll teach you how to use twin daggers, as well as illusion magic. To impress him, you should 1) volunteer to go first, 2) express that your ideal knight is flexible, 3) work alone, and 4) forget dodging.
The second choice is the most important one.