shy Shy children never laugh until everyone else does.
bashful The boy is bashful and doesn't talk much.
sheepish Ziri was talking with a sheepish air of apology.
diffident Tom is such a diffident man. He seems to have quite low self-esteem.
fearful “People are very fearful of the virus,” he added.

Popularity over time

Use in printed books, per decade from 1850 to 2019.

By length

3 letters: shy¹, coy¹
5 letters: scary², faint¹, mousy²
6 letters: afraid², unsure², craven²
7 letters: chicken², nervous², fearful², gutless², bashful²
8 letters: cautious², retiring³, backward², hesitant³, cowardly³, skittish², sheepish², timorous³
9 letters: tentative³, shrinking², spineless², dastardly³, diffident, shamefast
10 letters: deliberate⁴, meticulous⁴, submissive³, shamefaced
11 letters: intimidated⁵, unassertive
13 letters: pusillanimous⁵

The small number after each word is its number of syllables (scary² = 2 syllables).