How to unlock Kazumi:
Working on the Corporate job track, continue until you reach CEO. After impregnating Kazumi the first time, go to her and select the "Talk option." She will mention that you ought to join a Gym.
1. Start a new week. Work once at the office, then go to the gym, and after your workout, use the baths. You may need to repeat three or four times until you hit her max fertility period. A scene will occur in the baths.
2. New week. Speak to her in the harem menu, and she will mention that you should take a nap. Work one day at a time (don't do work all week), and you will eventually get an event offering you a nap. Take the nap, and you will get a scene.
3. New week, click on Kazumi in the harem menu. You can now use the phone app on her, which will give you her fertility cycle.
4. Don't fuck anyone for at least two weeks (or longer, wait until you get one of the wet dream scenes), then meet up with her during her most fertile period (%60). Ask to impregnate her, and she will tell you to meet her at the love hotel.
5. Repeat paragraph 4 more time. At the love hotel, she will ask you if you love her. Tell her yes (don't lie), and she will force you to impregnate her, thus unlocking Kazumi.