People from other tribes are like dark matter. They exist - in theory. They may even leave trace evidence behind. But for Narcissa, part of the Green Tribe (to Harry's Red), everyone she knows says they hate muggles and muggle-borns and muggle-lovers because they're stealing the magic and making spells a little bit more difficult for good, upstanding witches like herself, and the echo chamber in Green Tribe resonates with the Hogwarts Lake like even the merfolk are singing about how muggles ruin everything.

She will absolutely fight for Green Tribe and do what she can to keep Green Tribe going because the alternative is muggles ruining everything and there being no magic left to do magic with. Red Tribe says it's good for the community because the community is growing and accepting new ideas and fresh perspectives and helping to bring the wizarding world into modern technology what with cameras and playstations and all these other muggle toys, but their foremost expert on muggle artifacts wants to show you a collection of plugs that link into wall holes for some reason, and is inquiring seriously about a small yellow duck with no obvious advantages. Red Tribe shows you a device that makes still images that won't even wake up when prodded, and expects you (surrounded by family portraits that teach ancient wisdoms) to be excited for this silent, frivolous image capture. Red Tribe is adamant that these "muggles" offer something of value, as if the house elves just aren't good enough anymore - and they even have the gall to say there's nothing wrong with magic today when you know fully well that the old reliable killing curse didn't even kill a baby? And how do they explain that baby resisting one of the top wizards in the country - destiny? Fah!

Her problem is not that she abruptly wishes to join Red Tribe because Harry is in danger. Her problem is that Green Tribe has turned against Tribe Malfoy due to a series of unfortunate events (like Lucius failing to claim the prophecy, Draco being upstaged by Severus, etc) and she loves Tribe Malfoy more than being Green - even if that means wearing a different color for an evening.

But within the bounds of Green Tribe, she's still very much the "in" crowd - even if Green Tribe is about to fracture between the Emeralds and Jades or something (between milder and more extreme stances, that is.) She bleeds green and silver patriotically, except when it comes between her and her fellow Malfoys.