A person that is “diffident” (tímido) lacks confidence, or is shy.
The noun is “diffidence”, meaning a lack of confidence. Some people have a diffident (inseguro) manner. The adverb is diffidently.
Although Mr. Sancehz was diffident as a child, he was quite confident as an adult (aunque Señor Sanchez era un niño reservado, como adulto era bastante seguro).
His manners were mild (suave) and diffident (timido).
The employee was diffident in his first week in the new job. He behaved diffidently.
Do not confuse “difidente” with diffident. “Difidente” means untrustworthy (no muy de fiar/de poca confianza).