"His personal theory is important."
Translation:그의 개인적인 이론은 중요해요.
I just looked up 개인. It means 'individual'. I think that is the noun version. So 개인적인 means 'individual' in the adjective sense. I think a good synonym for the noun 'individual' would be 'person'. And a good synonym for the adjective 'individual' would be 'personal'.