Quinn is indeed not currently a LI as defined by the games mechanics, but she doesn't need to be - she is already, by far, the 2nd most important character in the game after the MC.
DPC's writing style is to use
a catalyst character to drive much of the plot forward. In BaDIK this is Quinn.
Right now, any LI can be taken out of the story as easily as Katherine Heigl was from Grey's. To much gnashing of teeth from their fans to be sure, but from a plot perspective only a sub-plot is lost. Not so Quinn.
Take C.5 for example. Render count wise, Quinn only had a cameo, However, the DIK party going sour, storming to the Jocks and then the Preps, leaving the mansion unattended to get thrashed. This
all stems from Quinn's decision to maneuver the HOTs to the Preps and then not tell the DIKs about it. Remove one of the LIs from C.5 means trashing a couple hundred renders (depending on the LI) but the main plot would still be coherent. Remove Quinn and there is no C.5 at all.