Emily had a rivaly with his next door neighbor Ralph, as they caught the same Pokémon. When they caught the Nidoran, they split them, even if they were in love. After their Nidoran were captured by Team Rocket, they set aside their differences and worked together.
Maria (on the left) belongs to Emily. He fell in love with Tony, but due to Ralph's and Emily's arguing, he was split from her, but continued to see her. When Tony and Maria defeated Team Rocket, maria evolved into a Nidorina.