Jessica and Rebecca are old friends and childhood playmates, though only because Jessica's family has worked for Rebecca's for generations. A fact that Jessica tries to hide so she can portray herself as a rich girl.
At first, Jessica thought of Ash as just another person, and may have looked down on him for his lack of a Pal prior to Echo's birth. That all changed when Ash saved her life, and revealed he was her crush the Silver Knight. Ever since then, she's been very attracted to him.
Jessica sees Silvia as something of a rival for Ash's affections, but seems to have grown to respect her.