In the movie, Clarisse's role is filled by Luke Castellan, who in the books is the son of Hermes and the leader of the villainous Kronos' army. This change was likely made to streamline the narrative, consolidate the villainous role, and perhaps to make the film's antagonist a more charismatic and relatable figure, as Luke was portrayed by the popular actor Brandon T. Jackson.
However, in the books, Clarisse is a significant character, known for her fiery personality, strength, and loyalty to her father, Ares. She is a fierce warrior who is not afraid to stand up for herself and her friends. While she might not be the most likable character at first, she ultimately proves to be a valuable ally to Percy and his friends.
The decision to omit Clarisse from the movie is a significant departure from the source material, and it is one of the many changes that the film made to the story.