Persephoni still has a lot to learn about being an Elite Guardian, as it seems like Eddy didn't tell her the entire job description from the start. On the same note, I've hinted in past comics about the true role of an Elite phrasing it more as a "security blanket" kind of deal but I never fully revealed why Chelsea was not allowed to know of the Guardians' existence. This page finally clarifies that part.
As a foster child, Chelsea has had a difficult time finding people she can connect with. If she knew that Persephoni had been assigned to her, she would be suspicious of whether or not she was ever actually her friend in the first place, or if she was "just doing her job." In keeping her in the dark, Persephoni first as Pearl, and now as Chloe can be seen as someone who honestly and sincerely cares about her. Whether in the form of a pet or a close friend, Chelsea needs a source of love in her life... and she needs to believe that it is real.
But, of course, the problem arises that Persephoni cannot be in two places at once...