na-adjective, noun which may take the genitive case particle 'no', noun
shy; reserved; withdrawn; introverted
Describes a person who tends to hold back, avoid asserting themselves, or withdraw from social situations. Often used for a personality trait rather than a temporary state.
彼女は引っ込み思案で、パーティーではいつも隅にいる。
She is shy and always stays in the corner at parties.
I want to overcome my reserved personality.
引っ込み思案 describes a reserved, hesitant personality that holds back from social situations in general, while 人見知り is specifically about strangers.