He has a plan

As soon as Erika entered the Pokemon Center, Ash had a plan. He paused her, then turned to his harem.

It was too risky to bring every woman, so he left Jasmine, Joy, Jenny, Viola, Misty, and Serena, bringing along Delia (in her slutty teen personality) and Jessie (in her little sister personality). He used Nurse Joy's keys, which he'd swiped off her when she joined his harem, to lock the room his girls were in.

"I hope this Center's Nurse Joy doesn't need to go in there," he said.

With that, he unpaused Jessie, Delia, and Erika.

"Ash Ketchum!" Erika said. "What a surprise! Are you here for a rematch?"

"Not me," Ash said. "This is my friend Delia. She's a rookie Trainer and she wants to battle you."

"Yes, please," Delia said. "You'll be my first Gym Badge."

Erika, who thought Delia truly was a novice Trainer, chuckled. "Well, Ash can take you to my Gym."

She turned to Jessie. "And who are you?"

"Jessie Ketchum," Jessie said. "Ash's younger sister. I'm a student at the Trainers School in Viridian City. We had an assignment to watch how different Gym Leaders battle, so I figured I would tag along with Ash while he helps Delia."

Erika smiled, not realizing this scantily clad schoolgirl was really a wanted criminal.

"Well feel free to come see Delia's battle," Erika said. "Prepare all you need, Delia, then meet me at the Gym."

Ash had Delia prepare by loaning her some Pokemon he had caught along the road. A Pidgey, Zubat, and Nidoran to be specific. He knew since it was Delia's 'first' Gym battle, Erika would go easy on her.

They then made their way to the Gym. Everyone in the city was so busy with their days they didn't notice the women dressed oddly.