Seto is too passive and panicky, understandable when you consider his slow self esteem...
But in key points he rises to the occasion (like losing his cool about the douche who called her a slut) and shows how much he cares about Anjou, this is why she loves him and want to be with him, with any other character the point where she lashes out on him would drag on and become drama, but he knows the moment is not about him and she only said what she said because she scared, he then proceeds to make her feel secure and wanted, like a good partner would...