Candace's biological father wasn't a bad man exactly…
She thinks about him occasionally, but it's like a movie playing behind her. She can hear the audio, but the picture is just not there. She knows that's still more than Phineas has of him, which she actually sees as a blessing for him.
Because she can still hear the tape from the night her mother told him she was pregnant with their second child.
"I didn't want the first one, Linda! What makes you think I want another one?"
She didn't sleep that night, and she didn't ask where he was the next day, or any day after for that matter. But when Phineas came home, she swore she'd be better. She'd make things easy on her mom and she'd help take care of him.
Then, a few years later, she met Lawrence.
She avoided him and his son as much as possible at first. Which was made difficult by the fact that he was always with her mom and Ferb was always with Phineas (or, rather, Phineas was always with Ferb; he was the more social).
But the longer Linda and Lawrence were married, the easier it became to see them, to talk to them, and even to pretend that they could be a family.
It was her ninth birthday when everything finally fell into place. Things had gotten extremely cozy in the house. Phineas and Ferb were officially inseparable (they had even made her a birthday card together). They had already adopted Perry. And Lawrence was making Linda ridiculously happy.
But Candace was, for the most part, still feeling out of place. Until her birthday…
Her mom had made her special breakfast. Then, Phineas and Ferb had presented her with their card. She almost teared up at the "we love you" inside, and pulled both boys in for a hug. Then Lawrence handed her a gift.
She unwrapped it slowly, peeled the lid of the box away, and gingerly reached in to pull out the fragile glass inside.
Candace gasped. It was the snow globe from the mall, the one with the unicorn and the girl she swore looked just like her.
"OH!" she exclaimed, grin covering her face. "I love it! Thanks, Dad."
There was a single beat when Candace's eyes widened and grin faltered, and Linda covered her mouth. Lawrence, with a small smile and glossy eyes, wrapped his arms around her.
"You're welcome, love. I'm glad you like it," he whispered, completely enveloping her in his embrace.
Candace only let another moment pass before closing her eyes, hugging him back, and smiling.
Yep. That was her dad, all right.