Lunar Coins appear to be a random drop from killing enemies. They are quite rare though; it is possible to go through some runs and never see one drop. If you do see one, grab it! The good news is that you don’t need to use it on that run. Lunar Coins are account bound, and you will always have them until you decide to use them, you don’t lose them when a run ends. This is important, as the Lunar Coins have many uses, and you will want to save up at least 10 of them to unlock the Artificer character.
Now, other things will try and tempt you to use your coins, but if you don’t have the Artificer yet, hold fast and keep saving those coins. The first thing you might notice dotted around the stages are these odd Lunar Plants. If you give them one Lunar Coin, they will open up and give you an item. I honestly don’t know enough about the items they might contain to tell you if this is worth it, I can simply remind you that you need lots of coins to get a character. If you want to spend them on Lunar Plants, that is up to you.
The Lunar Portal
The second thing that might tempt you is the idea of opening the Lunar Portal and visiting the Bazaar Between Realms. This is a strange shop with a stranger vendor, and it is from this odd chap that you can unlock the Artificer. If you give the vendor 10 Lunar Coins, you will unlock the character.
You need to find a strange blue rock, with light blue spikes, and give it one Lunar Coin. If you do this, the Lunar Portal will open after you defeat the stage boss. Do not use the teleporter, instead go through the portal to visit the Bazaar Between Realms. In there you can trade items for new items, buy Devil Deal Items that appear to bestow powerful buffs, but with a downside, or get the Mage if you have enough coins.
If you ever want to know how many you have, you can see the number just below your Gold counter in the top left corner of the screen!