Tatsugami Otohime

She is Kamijou Touma's cousin and is shown in the Angel Fall Incident. She does not show up again after the Angel Fall incident, in either the novels or the 1st season of the Index anime. She has more screen time and thus is sligthly portrayed better in the novels.

Her personality is playful and enjoys acting childish, she has stated that hugging Touma, clinging to Touma or flirtling with Touma (in public or private) is completely ok. She preferes to calls Touma Onii-chan.

The Angel Fall incident changed her outer appearance with Tokiwadai's Ace Railgun Mikoto, a fact that greatly disturbed Touma in more than one opportunity. He described her behaviour as that of a "little sister type" character.

ToAru Wiki