Len's growing love for Miku (Miku x Len)

Len's sisters name is Rin and Rins best friends name is Miku an she's known her ever since she was very young. When ever Miku was with Rin, Len would always be there because he's Rin's brother. Len always really liked Miku and felt comfortable around her but only thought of her as a friend up until now. When ever he's around Miku, his heart races and face turns red. Miku has also always thought of Len as a friend but slowly seems to be liking Len just a little more, what could this turn out to be? (This might have some smut in it)