What is a single word for a person who is underestimated in this situation:
a group of people underestimates/humiliates one of its members by treating her/him so bad, i.e. verbal abuse, harassment, rudeness and such.
Such a person is often called a pariah.
Originally the word referred to a member of a particular Indian caste. Nowadays, it is used to refer to anyone who is disrespected, ignored and is denied a chance to put their case.
Underdog fits what you are looking for:
An underdog is a person or group in a competition, usually in sports and creative works, who is not favored to win and popularly expected to lose. The party, team, or individual expected to win is called the favorite or top dog. In the case where an underdog wins, the outcome is an upset.