Hero units are created by specifying their first and last name in the barracks "Customize" screen. Once you do this, your soldier will be replaced by the hero unit. These units are considered to be cheats, though, as they are super-powered. They have high stats and start with advanced gear. You will be notified when you specify a hero's name that achievements are disabled as a result.
Also note that customization of these heroes is disabled - you can't change their names or any of the other options on that menu (armor color, armor type, gender, etc). You can also take their gear, but your other soldiers can't use it without doing the appropriate research.
You can summon as many clones of each hero as you choose. If you want to field a team of all Sid Meier clones, you are free to do so.
There are four confirmed heroes so far:
(I'm providing their nicknames for flavor, you don't have to specify them when you customize your character.)
These names are given in the game's files, you can find them in:
(Steam Folder)\steamapps\common\XCom-Enemy-Unknown\XcomGame\Localization\INT\XComStrategyGame.int
Search this file for "SuperSoldierFirstNames" and you'll come across all four, in addition to the strings that prompt you when you enter their name.
Here's Otto, note his 100 HP, Will, and Aim:
Kotaku also has a video guide which shows off summoning Sid himself, as well as Ken Levine.
These are what grant you XP and how much:
- Alien Kill 30/60*
- Regular Missions 60/90*
- Storyline Missions 120/180*
- Zero KIAs 20
- Resist Psi Attack 10**
- Successful Mindfray 20**
- Successful Mind Inspiration 30**
- Assist Mind Inspiration 30**
- Successful Psi Panic 30**
*Additional XPs earned if the soldier has a rank of Lieutenant or less and kills any of the higher alien ranks (Ethereals, Sectopods, Elite Mutons, Sectoid Commanders, etc.) during the mission.
**Psionic actions.
Your soldiers have different requirements for promotions:
- Rookie 0
- Squaddie 90
- Corporal 300
- Sergeant 510
- Lieutenant 745
- Captain 1100
- Major 1560
- Colonel 2150
- Psionic* 0
- Specialist 50
- Operative 120
Source: http://ufopaedia.org/index.php?title=Soldiers_(EU2012)#Earning_XPs
Best Answer
Panic is a one-shot deal. When a unit panics, they react erratically, and lose their next turn.
This means that, in a best case scenario, you will regain control of the panicked soldier on the next turn.
However, XCOM being what it is, your soldiers may panic again (the lowered will from having already panicked not helping), with the same unpredictable results.
Late game, the psionic ability "Psychic Inspiration" will remove panic, and re-allow them to take their actions. If cast preemptively, it can often prevent panic from happening in the first place.