@myshi Extra Guy probably is her stepbrother. He said that his mother has remarried and how that was almost regarded as "adulterous"; in the flashback Ai's stepmother was cause of strife to her for the same situation and she herself mentioned to Yuu that she has a son that is his same age, and you can see that she and Extra Guy have the same hair color. So it's easy to connect there.
For sure he is connected to Ai as he had a photo of her with Yuu and Tsubasa in his phone all along.