In Assassin's Creed Rebellion, you are constantly fighting during your mission. If you stand face to face with your opponent, you can expose your character to loss of health points and even death.
Your character may die after a heavy loss of health points. These, can be lost in combat or by improperly performing mission activities, e.g. if your character falls from a great height.
The hero cannot be reborn during the mission. In order for a character to join your team again, you can either heal them or wait for automatic health regeneration.
To heal your hero you have to go to the "Heroes" tab and click on the icon next to the hero's health (third picture on this page). Each medicine replenishes a given number of health points.
You can get the potions from chests or during a mission.
Can I lose my character forever?
In Assassin's Creed Rebellion you can unlock 46 characters. These are different types of heroes with different classes, skills and special attacks. However, you cannot remove any of them from your team.
Heroes HP regenerate automatically, so there is never a situation where the hero doesn't come back to life and leaves your team.