Chino originally came from the real world, and was teleported to the pokemon world at the beginning of our story.
When she was teleported to the pokemon world, she was changed in some ways: the most obvious being her age, which was reset from 24 to 10 years old. Her social anxiety, which she foolishly chose to keep in the pokemon world, was much more severe around humans than it used to be in the real world.
One bonus that was allowed in exchange for the social anxiety was her being teleported to the pokemon world *with* a *history* in the pokemon world. That is, Chino has memories of growing up in the pokemon world. She has familiarity with pokemon, the ins and outs of survival, and an (opinionated 10 year old's) understanding of how the world works. Had she chosen to be less of a socially anxious hermit, she may have had friends, family, and acquaintances. But the history that has been created for her is set in stone, and here it is:
Sure, Chino is still only 10 in the pokemon world. But a lot can happen in 10 years. Chino was born into a rich family, with a diplomatic father and a scientist mother. Her mother, Claire, researched renewable energy and the role of pokemon in a net-zero world, while her father Barry also lobbied for more renewable and sustainable sources to be used for energy. Of course, they met through their work, fell in love, moved into a rather large house, married, and had a single child: Chino.
The parents loved her, and brought her up with care. Because of their shared love of the planet and pokemon, they instilled their own love of pokemon and nature in their daughter, too. Early in her life, before she said her first words, it was clear that her communications with pokemon were fairly advanced. When they sent her to school, she struggled to make friends, but easily formed bonds with the pokemon in her life. Despite this, her parents loved her, and if anything were very proud of her special gift with talking and bonding with pokemon.
However, Chino was not so happy. She did not enjoy going to school; did not enjoy the other children her parents brought to her house for her to play with; and did not at all enjoy having to meet her parents' friends and high-society diplomats. The clothes she had to wear were uncomfortable, and she never knew what strangers were thinking or what to say to them. At school, the other children were so unpredictable and seemed to hide what they were really feeling, and with the adults it was so much worse.
At the age of seven, they had a family guest around. Claire's older brother, (or Chino's uncle), old man Dunc. There was a massive age gap between Claire and old man Dunc. Despite the head start in years Dunc may have had, Chino's mother was clearly more well-off in life. A well paying job in research, a grand house, and a diplomat husband. Meanwhile, old man Dunc seemed to not have a job, and be living "off the fat of the land".
He was dressed a lot more messy than all the other guests that would come to the house, which made Chino feel a bit more comfortable. Despite his messy dress, it was clear that Claire had a lot of love and respect for her older brother.
"Och, look at wee Chino! Ye've grown so much, ye know that?" Dunc said, giving Chino a friendly hug and ruffling her hair. She didn't remember meeting him before, but of course being so young, she didn't remember much from before the last year or so.
Well, something about this old man really made Chino feel at ease. He lived in the outdoors, with pokemon, and not with people, apparently. And Claire had invited him over to see how he would get along with Chino: if they got on well enough, maybe Chino could spend some time with him. A couple of weeks, camping outdoors... Chino would love it. Since they got along so well on their first meeting, Claire expediated the process and sent Chino straight with him after the dinner. Chino didn't need to take much stuff with her as old man Dunc was experienced and well-supplied himself, but Claire supplied her with her own backpack, sleeping bag, and some camping crockery.
Unbeknownst to everyone, though, was that Dunc had been doing his own research, and coming to his own conspiratorial conclusions about the work that Claire was doing. Chino stayed with Dunc, and neither of them returned after the two weeks was over. In fact, Claire had tried calling them during their camping trip, but Dunc's phone was off. Totally destroyed, crushed under a rock, in the next town along from Claire's house.
Chino was loving her trip. She was a bit surprised that she wasn't going back home, and that she hadn't been able to talk to her parents, though. She felt homesick for a couple of months, but after that, she was okay. She still loved her parents, but wasn't upset at not seeing them. It was more comfortable here, in the outdoors, and if she wanted to go see them, she'd have to go back to the house, which she didn't really want to do. She wasn't aware that her parents were worried about her, or that Dunc was effectively kidnapping her.
As time went on living with old man Dunc, Chino became perfectly capable of surviving on her own in the outdoors. Dunc was spending more time doing his own "adventuring" and less time at wherever their base was, as Chino became more independent herself. Although the both of them got along with pokemon, it could still be dangerous to be alone in the wild, so Dunc often left Cyndaquil to help look after Chino whenever he left base. He also gave Chino a small but heavy baton for self-defence, in case anything happened to Cyndaquil - he is a fire type after all, and there are lots of water types around. In their time together, Chino and Cyndaquil grew quite close.
So, yes, at some point when Chino was about 9 years old, Dunc never came home again. Presuming he had died, Chino took ownership of Cyndaquil, and eventually grew more confident in herself as the owner of all the belongings he had left behind. At the age of 10, she became merged with the real-life Chino.