where the weaker character needs to process whether he's a drag rather than a value add, don't always mesh with the necessary beats of a romance. Her relationship with her suitor therefore makes use of the tropes consequent to working together as a team to do violence. The tsundere archetype is defined by a relationship with a corresponding protagonist, but Shana is also obligated to fulfill the role of action hero.
Shakugan no Shana has the deep misfortune of needing to face the Perfect anime Toradora! when it comes to comparing main characters and their arcs.