Nanase is clearly in love with Fuuka. As you can see in chapter 17, Nanase blush slightly when Fuuka get close to her on the floor in the compartment. Not only that, but Fuuka might also have secret feeling for Nanase that she don't know yet. She is so focus on Yuni coz she is coping and doing the copium. She look to Yuni for comfort becoz that's all she know. I think at the end they will come together becoz of they shared feelings about Yuni and will end up fall in love.