Each character available in Assassin's Creed Rebellion has levels. The character stats depend on them. In the game you must constantly develop your character, so that they have a chance to fight tough enemies. If you don't level up your characters, you won't be able to perform the missions and progress in the storyline.
After achieving level 2, you'll unlock the Training Room. In this room the characters will be able to train, and thus increase their level.
In order for the characters to be able to train, you must click on the icon visible on the left side of the screenshot, and then select the assassin you want to train.
The next step is to choose the resources from which they'll draw their knowledge. You can learn with the help of Rare Codex Bundle, Codex Bundle, Codex Scroll or Training Codex. These resources have different power, so you choose them depending on how many levels you want your assassin to get.
Where will I get resources for learning?
After reaching level 10, you'll unlock the Library, this allows you to create Training Codices.
In the store, in the Resources tab you can buy the resources, whose prices are fixed:
1. 80 Rare Codex Bundle - 900 helix points,
2. 80 Codex Bundle - 200 helix points.
By completing missions you can get the resources needed to train the heroes.