Reserved, retiring, modest.

Of a person, or a person's character, behaviour, etc.: shy; unassertive; reserved.

= retiring, adj. 1. Now rare.

resembling an oyster; esp. (of a person) reserved or uncommunicative (staying inside one's ‘shell’).

Reserved, quiet. Obsolete. rare.

shy of oneself: unwilling to expose oneself.

Drawn in. as adj., or before noun/ˈɪndrɔːn/. Also figurative.

Of the mind or thought: Directed inwards upon itself, or upon that which is inward or spiritual. Also transferred of a person: Given to…

attributive or as adj. Keeping in the background; retiring. colloquial.

Of or relating to retreat (in various senses); characterized by retreat or reserve; retreating.

Psychology. A person characterized by introversion; a withdrawn or reserved person; opposed to extrovert, n. Also attributive and as adj.

Characterized by introversion (sense 1b).

In various senses of the verb; sometimes in a purely static sense, secluded; also of mental state, detached. Also spec. in Psychology, characterized…