Well, when Index is on a good mood she plays with Touma biting only his arms without causing pain in difference when she is angry, bites viciously Touma's head.
katekyo said: Question why does Terra appear in the preview anime? but not in the series?
I'm guessing they thought they could get through to volume 14. That or they originally planned on skipping parts of the story. Or it's just the fact it makes for good promo material.
