In truth there is not that much Nora. It's just that what there is escalates pretty fast and there's heavy implication of her getting pregnant. It pushes her above the level of Camila, Sarah and Mel, but she is still emotionally behind Rio, Quinn, Lily and Nicole. Jade also should be recognised as a serious SG. It's practically impossible for things to end well with her, but the relationship has very high stakes. It's a different form of serious.
On a tangent, it's interesting how the HOTs have this pattern of trios:
Main Girls: Sage, Josy and Maya
Serious Side Girls: Rio, Quinn and Lily
Casual Side Girls: Camila, Sarah and Mel
Girls for cucking a DIK: Ashley, Elena and Heather