Symbolism, Imagery, Allegory
Pop quiz: When is a boy wizard not a wizard? When is a boy vampire not a vampire?
Answer: When they're stand-ins for all the things that are lacking in your real life. Or, you know, symbols.
Sure Cath loves Simon and Baz for the characters they are, but she's tweaked them from the way they were originally written into boyfriends—each other's, not her own—in a relationship that's entirely her own design. In making these two nonthreatening children's characters into lovers, Cath is able to explore the romance she's never experienced. Even though Cath's never had a serious boyfriend (Abel doesn't count; she was just using him for the tres leches birthday cake), she can write Simon and Baz's love story all day long. As Wren says at one point:
Seriously, […] you know what love feels like. I've read you describe it a thousand different ways. (7.66)
In other words, while Cath may not get excited about real live boys until she goes off to college, she's been exploring the bodies and machinations of them through the fanfic she writes for a while. It's been a safe space for her to sort romance out, and as her fans clamor for more, she's validated in her assessment of love and relationship dynamics. For someone with serious trust issues who has a hard time integrating socially, the relationship between Simon and Baz that Cath's created has allowed her to test the romance waters without having to dive all the way in.
Importantly, as Cath opens up to Levi and begins spending more time with him (read: every second possible), she finds herself apologizing to her fanfic fans for not posting as often as she once did. As she allows herself to become immersed in a romantic relationship of her own, the one she creates between Simon and Baz falls a notch or two on her priority list.
Simon and Baz as Cath writes them aren't just symbols of Cath's relationship to romance, though—they also are stand-ins for some of her issues with her parents, particularly her mother. Cath tells Wren:
[…] I think Simon wants so badly for the Mage to be his dad that he won't let himself accept the overwhelming evidence. If it were wrong, it would ruin him. (12.106)
Hmm… let's take a look at that. Cath obviously wants—or wanted, anyway—to have Laura be her mom. What's the "overwhelming evidence" in Cath's case? That Laura has no desire to be a mom at all. Sure she shows up to spend holidays with Wren, but she doesn't come to the door, and though she shows up when Wren's in the hospital, she takes off before being allowed to visit her in her room.
As moms go, Laura's pretty useless, but unlike Simon who hasn't been ruined by his desired parental figure yet, Laura already did ruin her daughters. As Cath says, "I won't ever have any normal relationships—and it's always going to be because I didn't have a mother. Always. That's the ultimate kind of broken" (20.70). It's a big fat thanks for nothing, and while Cath doesn't talk much about her feelings (it takes her a super long time to tell Reagan about her mom, for instance… and they live together), she can express them through her fanfic.
There's one last level that Simon and Baz symbolically work on for Cath, and that's friendship, the kind Cath has trouble forming in the real world. Not only does she get to hang out with friendship through Simon and Baz, but online, she's a total stud. The real Cath may not have friends, but Magicath has legions of fans hanging on her every word. It's the only kind of popular she's ever been… at least, until Reagan and Levi come along and pull her out of her shell a bit.