You will earn rubies whenever you complete certain quests and when you level up. Free rubies are very scarce in this game, so you will to start saving up as soon as you start the game.

The earliest example of this is Wednesday Addams. You are given the opportunity to unlock her as soon as level three, but if you have been saving all of your rubies like we have, you will only have 55 rubies at this point in the game, and she costs a whopping hundred rubies to unlock.

You may be tempted to spend rubies early on to speed up certain process or buy the more exotic furniture, but resist the urge - you need to save up as many rubies as possible.