When President Joe Biden named Kamala Harris as his running mate, she faced questions of whether she would be loyal to him.

But 100 days into Biden's term, Harris has become one of the administration’s most prominent advocates for his agenda.

She stands alongside Biden at most of his major announcements and has built a relationship that aides say is closer than most presidents had with their seconds-in-command.

Harris has also taken on one of the administration's toughest challenges: addressing the root causes of migration to the U.S. from Mexico and Central America.

Harris' chief of staff, Tina Flournoy, told the Associated Press that the vice president has taken on the challenge "with gusto."

“She has said to me, ’When you decide you’re going to run for office and your name is on the ballot, next to it are not two boxes that say, ‘easy work’ and ‘hard work,’” Flournoy said.