From Bulbapedia, the community-driven Pokémon encyclopedia.
Marina (Japanese: マリナ) is a trainer from New Bark Town. Her starter was a Totodile (now a Croconaw), and she also owns a Jigglypuff and Misdreavus. Her Japanese voice actress is 村井かずさ Kazusa Murai, her English voice actress is currently unknown and her Brazilian Portuguese voice actress is Letícia Quinto. Her overall design seems to be based on of the female hero of Pokémon Crystal.
She appears in The Legend of Thunder!, where she does battle against Attila and Hun with the help of Jimmy, Vincent, and Eusine. She loves cute Pokémon and cute nicknames (Little Pink for her Jigglypuff, for instance), and does synchronized movement with them when she calls them out.
Her goal, unlike most other trainers, is to be a "Pokémon Idol." The specifics of being a Pokémon Idol are never made clear in the anime, but it seems to entail putting on a show while battling. Note that her first appearance occurred before the concept of the Pokémon Contest was even introduced to the series.
She also loves men in capes, especially Lance, having a whole notebook full of pictures of him. She flirts with Eusine because he wears a cape, and Vincent wears one to impress her.
When Jackson (Vincent) appeared in the Silver Conference, he referred to "Dani and Yoshi," leading many to believe that "Dani" was the official English name for Marina. Whether this was the original intention remains unknown. However, in The Legend of Thunder!, she is properly identified as Marina.
According to the same aforementioned conversation, Marina entered the Silver Conference along with Jimmy a full year before Jackson.
In Not on My Watch Ya Don't, Marina appears on the cover of a magazine, modeling the Pokétch. This may imply her goal of becoming an idol has come true. Additionally, she makes another cameo appearance in DP041, performing in a contest on Zoey's laptop.
Also, in the intro of Pokémon Ranger and the Temple of the Sea, she is seen running by the coast with Lugia and her three Pokémon: Little Miss, Wani-Wani, and Little Pink.