As a student, he was very quiet and introspective.
introspective
/ˌin-trə-ˈspek-tiv/
adjective
characterized by examination of one's own thoughts and feelings : thoughtfully reflective : employing, marked by, or tending to introspection
/ˌin-trə-ˈspek-tiv/
characterized by examination of one's own thoughts and feelings : thoughtfully reflective : employing, marked by, or tending to introspection
As a student, he was very quiet and introspective.