To unlock Celeste you have to give Kyu the "dirty magazine" (you can find it in your inventory). She will then give you a strange device in exchange. After that go to the beach at night with the strange device in your inventory (You dont actually need to finish the date, just leave the beach when you can).
When trying to leaving the beach, Celeste will appear and request the device back. After that she will be added to your HunieBee. Note: To be able to give Kyu the dirty magazine, you have to unlock Kyu First.
By Unlocking Celeste, you will also unlock the achievement:
To unlock Kyu you have to sleep with any girl. When you wake up the next day and try to leave your bedroom after sleep with your first girl, Kyu will appear and tell you that she wants to date you as well. She will then add herself to your HunieBee.
By Unlocking Kyu, you will also unlock the achievement:
To unlock Momo, you have to buy a "bag of goldfish" from the shop, then go outside (University Campus, Dawnwood Park, etc) during the day and drop (toss) the "bag of goldfish". When leaving, Momo will appear asking if she can keep the "bag of goldfish", she will then be added to your HonieBee. Note: The "bag of goldfish" isnt always avaliable to buy, just keep checking the shop every day and you will find it.
By Unlocking Momo, you will also unlock the achievement:
To unlock Venus, you have to sleep with all the 9 main Characters (including Kyu). When leaving the bedroom after sleep with the last girl, Kyu and Venus will appear and talk about your big achievements with the others girls. Venus will then be added to your HonieBee.
By Unlocking Venus, you will also unlock the achievement: