Despite being conceived by sneaky means, Celia turns out to be "good" in the same sense that Kevin is "evil." Eva has more realistic expectations for this birth; for one thing, while she doesn't expect a demon child this time, she also "knew better than to expect a blinding Vulcan mind-meld at her birth. A baby is a baby" (18.87). And though Celia isn't perfect, Eva loves her dearly.
Maybe Eva loves Celia so much because she is the opposite of Kevin. Celia tries hard in school, but fails; Kevin is intelligent, but doesn't try. Kevin breaks toys; Celia plays with the toys Kevin has broken. And due to an agreement with Eva regarding their last names, Kevin is a Khatchadourian, while Celia is a Plaskett.
Yet Franklin loves Kevin, and Eva loves Celia. Why? Perhaps both parents are afraid of how similar they are to the children who share their names. Eva loves Celia because she reminds her of what she loved about Franklin: his affection, his capacity for love, his gratitude. But Franklin sees these traits as weak. He calls Celia, "a doormat" (18.109), which is ironic, considering how he lets Kevin walk all over him.
Both Celia and Franklin are susceptible to Kevin's wiles. Celia "adored Kevin" and she "would believe anything" (22.135). That's a deadly combo when you have a sociopath for a brother. It's left to our imagination what Kevin does to Celia involving drain cleaner that causes her to lose an eye. We don't want to imagine it, but we do think it is entirely Kevin's fault.
Celia's biggest fault is that "she assumed that everyone else was just like her" (19.21). Franklin does the same thing. He thinks that Kevin wants the idealized version of a father/son relationship straight out of Leave It to Beaver that Franklin himself desires. Because Celia and Franklin are so similar, they suffer similar fates at the hands of Kevin: they're both surprised when he turns out to be a killer—their killer.