timid; fainthearted
彼女は気弱な笑みを浮かべた。
She flashed a timid smile.
彼は気弱になり、先輩に意見できなかった。
He became timid and couldn't voice his opinion to his senior colleague.
気弱な性格のため、彼はいつも人混みを避ける。
Due to his fainthearted personality, he always avoids crowds.