David Shield/Synopsis

History
While in Los Angeles, California, David and a group of scientists are rescued from an explosion by a young All Might, who had recently moved there from Japan. David thanked him and noticed his damaged attire. He developed a sturdy costume that could handle All Might's strength, which would later be known as All Might's Young Age costume. The two became close friends and acted as a hero team throughout California.

All Might shared his dream with David, telling him that he desired to become a Symbol of Peace that people can rely on. They took down a pair of L.A. Villains, and David relayed All Might's message to everyone watching, truly believing that All Might would become the world's top hero.

At some point, David had a daughter, Melissa, who was born Quirkless. All Might became an uncle figure to Melissa, and encouraged her to become a support engineer in order to help others, like her father does.

Eventually, David and his assistant Samuel would go on to develop and create a Quirk Amplification Device on I-Island, but their invention and research are confiscated and sealed away by their sponsors due to them being dangerous in the wrong hands.

It is implied that David kept in touch with All Might via email, and learned about his injury, not knowing about One For All or its properties. David offers to take a look at his old friend on I-Island.

My Hero Academia: Two Heroes
Some time before or during the events on I-Island, Sam convinces David to hire a crew of actors to pretend to be Villains so that he can steal back his research to restore All Might’s powers.

While working in his laboratory on I-Island, he is caught by surprise when All Might arrives, along with Izuku Midoriya, and Melissa, who invited All Might. David is ecstatic, but after noticing steam from All Might’s body, he asks Melissa to show Izuku around the expo. All Might reverts to his true form, and David is shocked to see the extent of his friend’s injury.

David uses a machine to scan All Might’s body, and is horrified to see that his Quirk levels have dropped drastically. David asks All Might why his powers have taken such a devastating drop, to which he gives a vague response. All Might then transforms into his muscle form to get ready for the I-Expo reception party, as David reminisces about the time he first met All Might. David openly fears the possibility of the world losing All Might, and that crime will rise up drastically.

David attends the party and listens to All Might’s speech until it is abruptly interrupted when the I-Island Security System sets off an alarm across the island. Wolfram and his crew enter the party and restrain the heroes present, catching the heroes by surprise. After Wolfram threatens to take the entire island hostage, David asks All Might to comply with the Villains’ demands. Wolfram receives orders to take a researcher, and attempts to take Samuel before David offers to go with him. Wolfram takes the two to the control room, where the two scientists proceed to steal back their research and Quirk Amplification Device.

The two are caught red-handed by Melissa and Izuku, who have arrived in the control room by fighting off the Villains. David admits to committing crimes and calling fake Villains, but explains All Might’s situation and that he was only trying to restore his powers. An angry Melissa shows her father a wound she got from a Villain, that she and her friends risked their lives to save everyone.

Confused, David says that the Villains weren’t supposed to harm people. Wolfram enters and reveals that they were real Villains the whole time. With Izuku restrained by Wolfram’s Quirk, Sam betrays David and brings the device to Wolfram. Sam claims that David turned his back on him and that they never got paid for working hard, but Wolfram double-crosses Sam and shoots him in the shoulder. Wolfram shoots again, but this time David jumps in the way and gets injured.

Wolfram mocks David’s heroism, reminding him that his crime won’t disappear with one good act. He knocks the scientist out and takes him, planning to use David to mass-produce the Quirk Amplification Device.

As Wolfram prepares to escape with the scientist via helicopter, Izuku pursues them. Wolfram threatens to kill David, forcing Izuku to stop. Wolfram continues to barrage Izuku with metal pillars, and prepares to shoot the young student, but David tackles the Villain and throws the bullet off course.

Suddenly, All Might, freed thanks to the efforts of Melissa, arrives and rescues David. As he tries to apologize, Wolfram traps David in a metal casing and attacks the No. 1 Hero.

After a long battle, All Might and Izuku go beyond their limits and defeat Wolfram with a Double Detroit Smash. David sees Izuku, and is reminded of All Might in his youth. All Might checks on David, who apologizes for causing so much trouble. David admits to fearing that the Symbol of Peace would fade away and lead to the end of peace. He realizes that he was hanging onto the past, and believes that Izuku and Melissa will protect the future and ensure peace to everyone.