Justice needs to be done

I wonder why Index always keeps freeloading off of Tōma and keeps biting him without a justifiable reason when the other party is at fault and never blames them. Although it's part of the plot and needs to be funny, she was also looking for Sphynx, finding him after he scared a crow into dropping a certain Indian Poker card. So... does anyone have an opinion on whether Index should be judged for what she keeps doing to our boy when he saved her at the cost of his memories?