From Bulbapedia, the community-driven Pokémon encyclopedia.
Serena (Japanese: サリナ Sarina), also known as Professor Serena, is a character of the day who appeared in Judgment Day!. She teaches in the Pokémon Battle Judge Training Institute, located on Bomba Island.
Jimmy met Serena when he accidentally arrived on the island while sleeping on a boat. Much later, he introduced Ash and his friends to her when they accidentally boarded the boat to Bomba Island instead of the one to Mossdeep City. Brock immediately fell head over heels with Serena but Max stopped him by dragging him away by the ear. Serena then agreed to enlist Ash and his friends as helpers to some trainees who were also becoming Battle Judges. Serena assigned Jimmy to become the judge of a mock battle between Ash and Brock. During the battle, Serena noticed that Jimmy made tons of mistakes with the judging such as when Jimmy mistaked Corphish for being defeated when in fact it just fell down for a few seconds. Serena explained to him that he was still learning and was expected to make a few mistakes every now and then.
Later, after Jimmy told his story to the group about the time he got Wartortle, Ivysaur, and Charmeleon, Serena returned and talked to Jimmy. She noticed Team Rocket with their mecha who used it to capture Jimmy's three Pokémon. Serena, noticing that Jimmy was disheartened when he fell of the mecha, told him about his good heart which renewed Jimmy's faith. After Team Rocket was defeated with the help of Jimmy's newly evolved Blastoise, Serena told Jimmy that he should become a Pokémon Trainer instead of a judge. The next day, Serena and Jimmy waved to the group her farewells after Brock did some flirting to Serena once again.