You all made me think ybout this issue a lot during the last few days, so I would like to share my thoughts with you.
In Hanako's route, Hisao makes this big mistake and tries to protect her from the very beginning rather than treating her as equal, which would mean not to protect, but to support her. This is what he is doing until they both confess their real toughts and feelings at the very end of her route. Therefore, I think Hisao's presence is useless for a long time. As long as he treated her, like Hanako said, like "a child", or "a broken person", he did nothing that really improved her condition. Only at rare occasions in Hanako's route, when he treated her as equal, for example when he first asked her about her scars, when he played pool with her at the Jazz club or when he showed her his scar, he had a real influence on her. Only wenn he decided to give her some space while she locked herself in her room at her birthday, THIS is the moment for me when her behaviour really started to take a turn for the letter. If Hanako did not realize her feelings, her love for Hisao by then, this was the moment she did for sure. So as not to lose him, she had to be more active. This is the reason she followed him into the city, showed him all her scars and shared a bed with Hisao. This, in my opinion, is all influenced by his behaviour. Although the reasons for Hanako's changes may have been very sad ones, and not intended like that by Hisao, they had an impact in Hanako. After their confession to each other, his influence on her reaches its top, with her motivation for tearing down her walls turning from "if I do not change, I will lose him" to "He really love me, I can rely on him".
In Lilly's route, however, he focusses his attention on Lilly. He is still there for Hanako, but I never saw him that over-protective like he is in Hanako's route. As opposed to her own route, she seems to trust Hisao at a very early stage of Lilly's route, seeing him as a true, SUPPORTIVE friend. Therefore, Hisao's influence in her behaviour starts way earlier, and is constant during this route. She gets more and more self-confident, even after Hisao and Lilly became a couple. Whether she fell in love with Hisao in Lilly's route, too, should not make a difference, and I'm very glad for that. It would have made me very sad to see her suffer from that.
So my conclusions are...
1. Does Hisao have an influence on Hanako? YES, in both Hanako's and Lilly's route
2. Is there a possibility that Hanako would have changed without Hiaso's presence? I don't think so, in both routes. The changes are because of his presence and behaviour. Maybe the would have occured later, or after meeting another person, but nobody can say that for sure.
3. Does Hisao have the same influence on Hanako in her own and Lilly's route? Again, I don't think so. While in her own route, his influence is near to zero at the beginning, except of the raw occasions mentioned above, but increases rapidly after her birthday. In Lilly's route, he has a constant influence on her, from a very early stage till the end. Furthermore, I think his influence in Hanako's own route causes far more and greater changes of her behaviour then in Lilly's route. In Lilly's, she gets the strength to work on her future, but still remains dissociated from other people. In her own route, she regains trust in other people, and is even able to kiss Hisao publicly. This is the route where she has the chance to come to terms with her past, leaving all the sadness and bad feelings behind.
"The reason I feel people are getting such Feels [while playing KS], is that the border between fantasy and reality is thin in this game. It feels real, and when you are done, its like spending a week getting to know someone intimately and then never seeing them again."