Rosa was a Ralts. But she was a "Pocket Pokémon," which could speak, had the intelligence of humans, and were tiny compared to both humans and even most normal size Pokémon.
Her father was a Stunfisk, and her mother was a Gardevoir. She had two brothers and two sisters, all of which were part of the Ralts evolutionary line. Every member of her family was a Pocket Pokémon.
Pocket Pokémon feared both humans and normal size Pokémon because they were massive to them. Rosa and her family lived in a village hidden in Viridian Forest. None of them had ever been to any place other than Viridian Forest, Kanto Route 2, Viridian City, or Pewter City.
One day, Rosa realized that her parents didn't love her. They were not abusive towards her or her siblings, but they didn't have time for any of them, so they ended up neglecting them and leaving them to develop on their own.
She considered running away and starting a new life, but she didn't know where to go. Despite her not knowing where to go, she took everything that she considered hers with her and ran away without leaving a note.
Rosa ended up in a clearing of the forest, having no clue where she was. She felt the ground shake and wondered if it was an earthquake. The shaking got closer, and she saw human feet just miss her. She felt good about avoiding being stepped on, but looked around and saw a human girl send out a normal size Typhlosion. She looked around and saw a younger human boy send out a Rattata.
She was caught in the crossfire of a Pokémon battle, but neither side paid any attention to the Pocket Pokémon because they didn't notice her.
For some reason, Rosa felt safer near the human girl because she looked older, so she ran to the Trainer's side, close enough to feel safe but far enough that she wouldn't be stepped on.
The battle ended fast, with the Typhlosion wiping the floor with the Rattata. The younger boy recalled his Rattata and walked away in defeat. The girl looked down after the battle, directly at Rosa.
The tiny Ralts had to crane her neck to see the girl's face. To her surprise, she did not give a hostile expression. That went against what Pocket Pokémon believed about all humans. The girl said, "I've never seen a Ralts that could fit in the palm of my hand."
She lowered her hand to Rosa, making it flat with the palm facing upward. Rosa said, "H-how do I know I can trust you, giant girl?"
The girl said, "You can talk," and Ralts was upset that she stated the obvious. She said, "I saved the Johto region from Team Rocket. I'm also the Champion of the Johto League, and I'm on my way to get all eight badges of the Indigo League as well."
Rosa didn't know what Team Rocket, Johto, the Johto League, or the Indigo League were. She said, "Well, I don't know where to go. I guess I can go with you."
She got onto the girl's hand, who lifted her. Rosa said, "What's your name, giant girl?"
The girl replied, "First, I'm not giant. You're just tiny. But to answer your question, I am Lyra. Do you have a name? If not, I can give you one."
"I do have a name. I'm Rosa."
"Want to go in a Poké Ball?"
"No!"
Lyra was surprised that this talking Pokémon didn't want to go in a ball, but she accepted this and said, "I've heard of humans and Pokémon allying without the former catching the latter. Maybe this will be like that?"
Rosa, still being held and not knowing what Lyra was talking about, said, "I guess. Can I go on your shoulder?"
Lyra nodded and put her hand near her shoulder. Rosa jumped onto it, then sat down. She said, "I like being here. May I stay on this part of your body?"
"Of course!"
Lyra continued her journey with Rosa on her shoulder. She had to make sure the Pokemon she had caught wouldn't try to eat the tiny Ralts.
At one point during the journey, Rosa asked Lyra, "I'd like you to take me on a tour of the rest of both Kanto and Johto. I never left Viridian Forest until you found me."
Lyra said, "Of course! Where do you want to start?"
"How about your home town?"
Lyra called out a Flying-type that was large enough for her to ride, then flew to the Indigo Plateau. She crossed into Johto, then called called the same Flying-type to take them to New Bark Town.
When they landed, Lyra said, "Do you want to go see Professor Elm? I think he'd be interested if he learned of tiny Pokémon that could speak."
"Please don't! I don't want to be separated from you, since you care for me more than my parents did!"
"Oh, yeah. I was wondering where you came from."
"I came from a village in Viridian Forest. It is inhabited by nothing but Pocket Pokémon, which is what Pokémon that are around my size call ourselves,"
"Since Pokémon means 'Pocket Monster,' would that make you 'Pocket Pocket Monsters?'"
"I guess. Anyway, my mother is a Gardevoir, and my father is a Stunfisk,"
"That's an… odd couple. I never realized that those two could breed. I mean, people often say that a Skitty and a Wailord can breed together, and while that sounds funny, it's true."
Rosa didn't know what those two were, but she said, "I have two brothers and two sisters, all of which are part of my mom's line. My parents weren't abusive to any of us, but they didn't love us, either. They didn't hate us, but they didn't have time for us, so we developed with little influence from them."
"That doesn't sound good. Do you like me more than you do your parents?"
"Absolutely! You pay attention to me and protect me."
Lyra felt good, but Rosa said, "My parents told me stories that Pocket Pokémon who were found by humans were either taken away, never to be seen again, or were used as food. Now, they never specified whether the humans ate them or whether they were fed to big Pokemon."
Lyra was upset at this, and Rosa said, "But if you wanted me dead, you would have done so already… right?"
Lyra shook her head and said, "I promise I won't hurt you."
Rosa said, "Now I want to see more of Johto and Kanto."
Lyra said, "Oh, right!" and called her Flying-type to fly to Cherrygrove City.
Lyra showed Rosa the rest of the cities and towns in Johto and Kanto because the two regions formed a continuous landmass.
By the time Lyra got all of the badges in Kanto and defeated Red, the Indigo League Champion, Rosa had evolved into a Kirlia. Her personality remained the same, but evolution granted her the power to distort the air around her to create illusions and see the future, which proved helpful in Lyra's battles.