I edited my original post for (hopefully) clarity. I mean both Main Girls and Side Girls are LIs because they have a path and are (at least potential) girlfriends. This means I include Quinn as an LI. Quinn is not a MG though (although in any other game, I'd probably call her one). She's as close as you get here, but she isn't an MG - and yes, it comes down to semantics and game mechanics, where she doesn't have a dedicated path in game terms (is part of the "others" path), doesn't have relationship points, is specifically outside the list of 5 MGs DPC has stated, etc.
Quinn is the most prominent girl in the story outside of the 5 MGs, but she isn't intertwined in the canon story like the MGs are. If you're not on Quinn's path, she isn't as present in the story as the MGs are - still a lot more so that any other SG when you're not on their path, but a level below the MGs. And if you are on Quinn's path, you're still spending a lot of time with and attention on the actual MGs, to a level not comparable to Quinn when off her path. All of this bit is irrelevent in the "is Quinn MG?" discussion though because in this game being an MG is a set thing as noted in my first paragraph.
If you really want to go down the "6th Main Girl" discussion route, and of course I say this with tongue firmly in cheek, then Derek is more MG than Quinn. Derek has relationship points, is integral to the canon story, and is one of the three options for Ep4 when you need somewhere new to stay.
TLDR: Derek is the 6th MG.
