reserved, self-effacing, unemotional, undemonstrative, taciturn, untalkative, unassertive, restrained, irreticent, untalkable, more...

outgoing, talkative, gregarious, extroverted

shy, reserved, introverted, quiet, withdrawn, hesitant, uncommunicative, tight-lipped, more...

little, first, singularly, more, unwontedly, distinctive, esoteric, tenuous

they're reserved,   shy manner,   being reserved,   saying little,   reserved, in a way,   shy and reserved,   inclined to silence,   unwilling to speak,   holding back words,   never say