Our character is the product of many factors. For me: my parents, my teachers, my sister, people I’ve met along the way and even an occasional role model found in literature or cinema have all contributed to who I am today.

I can remember at age 15 watching with rapt attention the movie, “To Kill a Mockingbird.” In it, Gregory Peck plays a widowed attorney (Atticus Finch) in 1930s Alabama trying to raise two young kids as best he can. A local black man, Tom Robinson, is falsely accused of raping a white woman. The local authorities visit Atticus at his home and beg him to take Tom’s case. The truth is that no one else will. Finch, at his own peril, takes the case.

At one point, Atticus finds himself on the steps of the jail (the sheriff had been called away on a ruse) confronted by an angry mob dead set on lynching Mr. Robinson. Risking his life and limb, Atticus stands up to the mob and largely due to the innocent intervention of his young daughter the crowd disperses.

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I have often thought of the jailhouse scene as a litmus test for our character. If we found ourselves in a similar set of circumstances, where would we place ourselves? Would we have the courage to face an angry, hate-filled mob? Would we be willing to die for our principles? Perhaps that is why I find any politician’s appeal to hate and mob mentality so appalling. That scene alone inspired me to dedicate my life to helping those who have no one else, the powerless, and the forgotten. But I’m not the only one.

In 2006, when I was running for Congress in a crowded Democratic primary field, a young man named Amadou Ly was equally busy. He was building a robot for his East Harlem High School’s entry to the citywide robot-building contest. His entry won and the next stop was the national competition in Atlanta. But Mr. Ly was not allowed to accompany the team.

Amadou Ly’s story is a complicated one. He was brought here from Senegal as a little boy in 2001. After his visitor’s visa expired, his mother abandoned him. In 2004, the car he was in was involved in an accident and the police ended up reporting him to the immigration authorities. That’s the bad news. The good news was that the lawyer assigned to him was a colleague of mine at The Legal Aid Society, Amy Valor Meselson.

Amy tried every possible legal avenue to help the young student. If one path failed, she tried the next. His trip to Atlanta was lost, of course, but now he was facing the bigger problem of deportation. The Dream Act was stalled and so Amy tried the court of public opinion. She contacted The New York Times, which ran a front-page story on Mr. Ly, resulting in a huge outpouring of support, both legal and political. Eventually, Mr. Ly was granted a student visa. He later graduated from college, became an American citizen and moved to Hollywood to pursue an acting career.

If there was ever a lawyer who lived up to my ideal of a dedicated and resourceful advocate of the Atticus Finch mold, it was Ms. Meselson. She was always a fighter, having suffered through a life-threatening respiratory disease in her youth, which made her middle name all the more appropriate. A graduate of Brown University, she received a master’s in philosophy from Harvard and eventually a law degree from Yale. With those credentials she could have made a fortune with the firm of her choice but instead she chose to work for my organization, the Legal Aid Society. There, she advocated ferociously for 14 years (2002-2016), helping those who had no one else to turn to. And help them she did.

During her tenure at the Legal Aid Society, Amy defended hundreds of clients, most of them children. Her specialty was defending unaccompanied migrant children. It wasn’t easy trying to run the legal maze of immigration, child welfare, and national security bureaucracies. Yet, she did it and she did it astoundingly well. Amy even went beyond the four corners of her job, recruiting volunteers from corporate law firms to represent foster children. It was her inspirational lobbying that led to the formation of the special juvenile section of the immigration court.

Ms. Meselson left my organization two years ago. Never shying away from heart-wrenching causes, she flew to Greece to volunteer at a camp for Syrian refugees. More recently, she became the managing attorney of the Immigrant Justice Corps, a volunteer group of lawyers doing the same work she had done for a decade and a half.

None of us are supermen or superwomen. My former colleague, Amy, was no different, suffering from depression, attention deficit disorder and extreme anxiety. Three weeks ago, she took her own life in her Manhattan apartment, and family, friends, colleagues, and former clients, like Mr. Ly, are all heartbroken.

Chief Judge Robert Katzmann of the United States Court of Appeals for the Second Circuit is quoted as saying this about Ms. Meselson: “a life saver and life giver…What Amy did was to give hope to immigrants and their families, to make it possible for dreams for a better life to be realized, for despair to be transformed into hope.”

Mr. Ly, now 30, had this to say about Amy: “I was able to stay in this country, I was able to live my dream and grow up and feed my family and help others because she helped me and she did it with open arms. She was my hero.”

It’s truly sad that this bright light has left us. Her example of selfless devotion to those most vulnerable remains. Her level of dedication will forever remind me of a quote from another fictional character, Tom Joad from John Steinbeck’s “The Grapes of Wrath,”—“whenever there’s a fight so hungry people can eat, I’ll be there.”

Amy Valor Meselson accomplished many things in her life, but she will be remembered most of all for her selflessness. Through her deeply committed and caring advocacy on behalf of those most vulnerable, she set a standard of character that we can all strive for. I know I will.