Célestine is introduced as the undisputed best Pokémon Trading Card Game player at Ava's new school. She carries herself with a confident, intimidating demeanor, openly mocking and belittling Ava's efforts to the point that Ava nearly avoids the Regional Championships altogether. Célestine's partner Pokémon is a Mismagius, reflected in her deck which specializes in Psychic-type cards.

A significant shift occurs following her defeat by Ava at the Regional Championships. Célestine transitions from rival to supportive friend, congratulating Ava on her victory and offering a respectful handshake in a display of sportsmanship. She joins Ava's circle of friends and provides steadfast support in later competitions.

When Ava loses most of her cards, Célestine demonstrates her loyalty by sacrificing her own collection. Alongside Joshua, she trades her cards with others to help Ava rebuild a functional deck for the International Championships. This act underscores her commitment to their friendship.

In the narrative's later stages, Célestine remains a supportive figure, accompanying Ava to the Pokémon World Championships. Her character arc illustrates a transformation from a competitive adversary to a gracious loser and a reliable ally.