[Update]: Slay the Spire devs are working on a fourth and entirely free DLC character!
After spending a rather considerable amount of time on the beta servers, Slay the Spire's third character "The Defect" has now finally made its way to the live version as well. Since this is Slay the Spire we're talking about, it shouldn't come as much of a surprise to hear that The Defect is not your standard mage character, but rather more of a robot wizard that needs to juggle orbs in order to unleash his spells!
There is a bit of a difficulty curve to learning The Defect's mechanics given that you can cycle through your orbs in order to change up your tactics, but also maintain only a limited amount of them at the same time. Furthermore, each of the orbs provides both passive and active bonuses, so careful planning is greatly encouraged if you want to get the most mileage out of your cards. Most importantly though, The Defect is a whole bunch of fun to mess around with, so I'd definitely recommend spending a bit of time getting familiar with its mechanics.
In terms of pure content, however, you're looking at 75 new cards (many of which have unique mechanics and keywords) with this update, 9 new and seemingly quite powerful relics, 3 new achievements, and even a bit of extra lore in the Sensory Stone event. The details are in short supply, but you can learn more about the update over here.
The final thing worth mentioning is that The Defect will be the last character released during Slay the Spire's stay in Early Access. Naturally, there are going to be more characters coming in the future, either as paid or free DLC, but right now the developers will be focusing on polishing and balanced the existing content in order to make it ready for launch. According to the recent developer post it took them a year to balance the first two characters, so hopefully history will not repeat itself with The Defect.
You can learn more about Slay the Spire, or just grab yourself a copy, by heading over to Steam. Have fun!