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i think the reason manaka chose nishino is because of the accident with toujou brother he thought that toujou already got herself a boyfriend and than there is no chance with toujou than he saw nishino and decided to go out with nishino maybe he choose nishino due to jealous of toujou brother accident he didn't think twice he saw nishino and just ask her out .....When manaka saw toujou during the fair thingy he let go of nishino hand quicky which i assumed that he still like toujou a lot but already went out with nishino and he just love both of them too much to hurt one or another feeling and already is going out with nishino again so he decided to close up his feeling for toujou and move on
so sad ..........Toujou to the end!!