Disclaimer: I don't own anything.
Secret Past and New Beginning.
Takato is actually the younger brother of Tai and Kari, Ryuji. He fought Millenniumon alongside Ryo and Ken. In the battle, WarGrowlmon was deleted, and the explosion struck Ryuji on a cellular level, causing him to somehow age until he was the same age as his sister. Ken went home to his family, secretly blaming himself for Ryuji and WarGrowlmon fates. Ryo lied to Tai and Kari, and said that Ryuji went missing after the battle. Matt, Sora, Mimi, TK, Izzy, and Joe never found out that Ryuji had become a digidestined like his siblings, or that he had a crest of his own. The crest of darkness.
Ryo then returned to the digiworld and took Ryuji to his hometown of Shinjuku. He helped Ryuji decide on a name and background story to get adopted. They decided on Takato, a child who couldn't remember his family or last name. Takato got adopted by Mie and Takehiro Matsuki. Ryo sent Ken an E-mail, including Ryuji's new information to keep in touch. After a little while, Ryo went to the digiworld because of suspicions about Millenniumon's return, but they didn't hear from him again.
One day, he found a blue card in his card pile, and he swiped it; causing his card reader to change into a D-arc. When he got back home, he swiped an old picture he had drawn as Ryuji. It was an age appropriate sketch of digimon and partner. He had one thought on his mind; being reunited with Guilmon. The screen flashed and a digi egg appeared. 'Just wait, Guilmon. We'll see each other soon.' Takato thought.
The next day, Takato got home from school and noticed that the digi egg on the screen had hatched. He pressed a button and the screen shifted to a radar screen. There was an arrow pointing in one direction. Takato left the house and tracked down the signal. When he got there, he found a small lizard digimon sleeping on the ground.
"Gigimon?" Takato said. The lizard's ears twitched as it woke. "Ryuji?" he asked, and then his eyes locked on Takato. "Are you Ryuji's older brother?"
"It's me, Ryuji." Takato said. "Ryuji? What happened to you? I knew you acted older than your age, but I think this may be pushing it a little." Gigimon said. "After the fight with Millenniumon, the explosion hit me, and it must have messed with my genes, because when I regained consciousness, I looked like this." Takato said.
"How did your parents take it?" Gigimon asked. Takato froze. "They didn't. Ryo made a cover up for me so I could disappear. He still helped me keep in touch with Ken, but he disappeared a few months ago," Takato said.
"Is your name still Ryuji?" Gigimon asked. "You can still call me that when it's just us, but everyone else except for Ryo, Ken, Monodramon and Wormmon knows me as Takato Matsuki." Takato said.
"Okay Ryuji. Can we go home now?" Gigimon asked. "Sure." He said, and they walked home. Takato snuck Gigimon past his parent's watch and into his room. After dinner, Takato slept happily with Gigimon.