There are five heroines: Umi, Shiori, Riho, Sango, and Nagisa.

Nagisa's route is unlocked upon completing at least one route.
A bonus "3P" route will be accessible on the main menu upon completing at least one route.
Contains two H-scenes: one for Protagonist x Shiori x Sango, and another for Protagonist x Umi x Riho.

Route Guideline

Kouzaki Umi
Help Kouzaki
Help Kouzaki or Shiori-san
Help Kouzaki
Return to the room with Umi and decide together
 
Maki Shiori
Help the others
Help Kouzaki or Shiori-san
Help Shiori-san
Ask Shiori-san about her plans
 
Kitsuki Riho
Help the others
Wait for Sango-chan and Riho-san
Wait for Riho-san
Invite Riho-san on a stroll
 
Tsukumi Sango
Help the others
Wait for Sango-chan and Riho-san
Go with Sango-chan
Follow Sango-chan and her lil' buddy
 
Kariu Nagisa
Unlocked upon completing at least one route
Help the others
End up getting instructions from Nagisa-san
 
Attribution

This KoiRizo walkthrough was created by Eclipsed, based on info attained from http://seiya-saiga.com/