Mana does have a locket containing a picture of her and Shido when they were younger, which is how she recognises him when she coincidentally sees him in a video. What makes this complicated is that both Shido and Mana have amnesia and don't remember their pasts. Date A Live has Shido and Kotori (the former being adopted into the latter's family) suddenly encounter the former's younger biological sister Mana one day.