"Red: Your dweller is sad, cheer them up."Yellow: Your dweller is doing okay, but could be happier."."Green: Your dweller is happy, way to go!".Happiness icons are color coded, for easy reference: Assign them to the training rooms for some book worm or physical training. If they are sick, a little RadAway and/or stimpaks will increase their happiness.Whether you’re a newcomer or a long-time fan of the Fallout universe, this Bethesda installment has a lot in store for you. If they are in any other room, their happiness will raise to at least 75%. Equip them to explore the Wasteland, collect rare items, defeat various enemies, complete quests, grow your population, and protect your home. It will lift their spirits and make them happy.
Overall it is best to have a well rounded special character exploring (as it says in the game) and a higher level character corresponds to more health in general. What has been said about endurance, strength, and luck are true in my experience. I assume that charisma impacts the frequency of encountering non violent scenarios that give you xp ( like when you patch up a run away slaves leg.) Intelligence/perception may also play a role in your ability to defeat harder enemies, like murlocks without taking much damage if any at all.
I have a hunch that agility improves your chance of escaping unscathed from a tough battle ( particularly the longer you are out in the wastes).