This page of our guide to Desperados 3 explains how to heal a wounded hero and how to renew a red health bar of a particular character. This is especially important on high difficulty levels, where you can easily get hurt and die if your hero is already injured.
In Desperados III, there is no single universal healing method that is available for all characters. Instead, you must use the active skills of the selected playable characters.
Doc McCoy has bandages that can be used to renew 2 sections of the health bar. An important feature of this skill is that it can be used to heal any character, not just McCoy himself.
Hector has The Good Stuff canteen and drinking it can fully regenerate his health bar. Unfortunately, this skill is reserved for Hector only.
Isabelle has healing herbs that can be used to renew 1 fragment of the health bar. Similar to McCoy, this skill can be used to treat any character, not just Isabelle herself.