Should Stoic Be Capitalized?

Definition 1 is actually lower-case stoicism. It's the modern-day concept of a personality trait or coping style, which people typically equate with having a “stiff upper lip” or the advice to “suck it up”, and so on. When used in this way it's never written capitalized.

Is it stoical or stoic?

As adjectives the difference between stoical and stoic

is that stoical is enduring pain and hardship without showing feeling or complaint while stoic is of or relating to the stoics or their ideas; see stoicism.

How do you use stoic in a sentence?

His personality is stoic to the point of being cold, even towards his younger brothers. Despite her stoic mannerisms, she is still quite playful and likes to be with her friends. He's calm and stoic, but he has a strong sense of justice in his heart.