Cassie Lang was born to Maggie and Scott Lang. Her father was a petty thief who vowed to quit his life of crime when his daughter was born; however, he was arrested after breaking into the house of the head of Vistacorp and sending back money the company had stolen from its customers. Cassie's parents divorced while Scott was still in prison and Maggie began dating Jim Paxton, who raised Cassie like his own daughter.
- "I'm not going to miss my little girl's birthday party."
- ―Scott Lang
A few years later, Jim Paxton hosted Cassie's seventh birthday party. Scott Lang arrived, having been released from prison. The father and daughter hugged and Lang presented her with a gift of an ugly rabbit toy which she loved. Cassie went to inform her mother of Scott's arrival, which she seemed to be excited about, spitting out her drink at the news. Maggie and Paxton spoke to Scott and, after talking in private, Scott left the party, waving goodbye to his disappointed daughter.
- "Mommy, is daddy a bad man? I heard some grown-ups talking. They said he was bad."
"No, he's not bad. Daddy just gets confused sometimes."
- ―Cassie and Maggie Lang
The night after the party, Cassie was being tucked into bed by her mother, who asked if she really liked the gift her father had given her; she assured her that she did because she liked its incredible ugliness. Cassie asked her mother if her father was a bad man because of the way she and Jim Paxton had spoken to him, but Maggie assured her that he was not, just complicated and easily confused.
- "Are you trying to find my daddy?"
"Uh, yeah I am, sweetheart. I just want your daddy to be safe."
"Hope you don't catch him."
- ―Cassie Lang and Jim Paxton
Cassie joined her mother and Jim Paxton for dinner and they discussed Scott Lang, who had just broken out of prison with Paxton given the task of tracking him down. Cassie asked her stepfather if he was going to find her dad; Paxton told her he only wanted him to be safe. In response, Cassie told him that she hoped he did not find him.
Late Night Visit
The night before he undertook a new top-secret and highly dangerous heist under the leadership of his new employer Hank Pym, Cassie's dad, Scott Lang, put on the Ant-Man Suit and snuck into Maggie Lang's house, going into Cassie's bedroom and looking at her while she was sleeping. Happy with seeing his beloved daughter again, Scott continued to watch her for a while before enlarging himself and kissing her forehead. Once he was satisfied with his moment of seeing his beloved daughter, he left.
- "Are you a monster?"
"Do I look like a monster?"
"I want my daddy."
"I want your daddy too."
- ―Cassie Lang and Yellowjacket
Without warning one night, Maggie Lang's home was attacked by Yellowjacket while wearing a weaponised Yellowjacket Suit; Cross threw Maggie out the house and took Cassie hostage, telling the terrified child that he wanted to find and kill her father. Lang also questioned if Yellowjacket was a monster, something which the supervillain did not deny.
Coming to her rescue, Scott Lang arrived moments later in the Ant-Man Suit and battled Cross, both shrinking down to minuscule sizes and fighting on Cassie's toy train set. Cassie watched in shock as tiny explosions occurred on her train set where the two fought and hundreds of Ants arrived and attacked Yellowjacket seemingly under Scott's command. Even more shocking was when Scott used some kind of device which caused her train set to instantly grow to gigantic sizes as well as one of the ants, which grew to the size of a large dog.
When Jim Paxton arrived, he tried to bring Cassie to safety, but Yellowjacket cornered them and attempted to kill them both. With only one chance left to save them, Scott Lang then shrunk down to the smallest size possible and attacked Cross, shrinking into his Yellowjacket Suit and damaging its internal mainframe, causing the suit to crush into itself and kill Cross and wipe him out of existence forever. For a moment, Scott was lost to the Quantum Realm, unable to stop shirking, however he was able to repair his own suit and return to his original size. Once back in reality, Cassie hugged her father tightly.
Months later, Cassie has adopted a giant ant as her new pet, treating it like a dog. Jim Paxton and Maggie Lang invited Scott Lang to their home for dinner to show their gratitude for everything he had done to protect their family. Paxton even informed Lang that he would not be going back to jail after breaking free the last time. Lang then received a call on his phone telling him to meet with Hank Pym and made a quick exit, kissing Cassie on the head as he left.
Cassie Lang is usually happy and oblivious to most things and, like most children, she loves her parents without a second thought. She loves her father, Scott Lang, knowing he is a good man at heart. Cassie also appreciates her step-father Jim Paxton, despite she once stated that her father would be safer if the police did not find him.
- Giant Pet Ant - Pet
- Darren Cross † - Kidnapper
- In the comics, Cassie eventually becomes the superhero known as "Stature" and wielded the same powers her father had, and was a prominent member of the Young Avengers.