You need to talk to Sana Chiba in Chapter 2 to unlock the romance path in Chapter 3

[Setsugekka]

Make sure to chat with Sana Chiba in Chapter 2 so you can unlock the romance option in Chapter 3.

[gungadin22000]

No thanks, crazy cat lady is my one true love.

(Just out of interest, what happens if you start romancing multiple characters at once?)

[cniudae]

Sana and the doctor will break up with you, the others don't care