Ishmael doesn't open up to his friends or other staff members in the book, only Esther. I think that because she is so kind to him and has taken care of him when he has fainted that he is forming a bond with her. She gives him presents such as a cassette player and a Walkman cassette. This is the first personalized present that he has gotten since he has been out of the war and possibly even before the war. Esther also takes him into the city and schedules for a check-up at the main hospital for him. I believe all of these things she is doing for him makes him feel like he can connect with her and that she is really trying to form a bond/friendship with him.
I believe Ishmael feels that she is the only one who has truly shown concern specifically for him and that makes him feel better and more human than a like a soldier. He had a dream about his family for the first time and it was really bothering him. After the dream the next day he went to see Esther and he felt comfortable enough to tell her about it. When she said, "This is not your fault," for the first time he actually started to believe it whereas before he hated it when the staff members would say that. He began to feel like he wasn't being judged by her and she really made him feel like he was only a child.
In my own life recently I came into a conflict at work. Needless to say I vented to the wrong person about my job and it got back to my manager. We hadn't really connected before and I thought I was for sure done for but she was kind about it and related it to the fact that she has a daughter the same age as me and her daughter has said similar things that I have. She treated it with such a kind and motherly nature and made me feel like I could trust her with anything if I were to ever have any problems again. It was comforting to know that she really cared for me as an employee and I wasn't just another worker who burdened her by my venting about how hard my job can be.
The cheerleader trusts her squad not to drop her.