Otto is Atsu's darling little brother; she adores him. When he was brought home, she instantly knew she wanted to protect him. She saw how small and frail he was, so she did everything with him. They've grown distant as time passed, simply because their social crowds are on completely different sides of the spectrum. Atsu is worried Otto resents her now, since he never really talks about his feelings. She hopes they can spend more time together in the future.
Otto thinks rather highly of Atsu, but has trouble expressing his feelings. He never ceases to be amazed by how socializing seems to come as natural as breathing for her. He's a bit jealous of her, but at the same time, he's also jealous of her friends for taking up Atsu's time, when she could be hanging out with him like she used to when they were younger.