Were do I begin?
Well I have beaten the game (All missions "S".)
First I will go through on how to unlock characters.
Lieutenant Commander Garret Schimitzer:
Complete the game once. The reuglar 13 missions once. He should be unlocked and playable in the simulation missions.
Get an "S" rank on the first simulation mission.
Hint: Fallow Char, once you get behind the enemy units strike them from behind to unlock him.
Get an "S" rank on the secound simulation mission.
Hint: Obtain as many blue kills as possible.
Complete "Trial Attack 1."
Black Tri Stars:
Complete "Trial Attack 2."
Obtain 3 "C" ranks or below in the Federation Simulations.
To unlock Lt. Agar, Kai, Hyato you must beat the mission "Attack Jaburo," on Ace mode (Beat the game once then play again. You will notice most of the type 61 tanks are GM[G] or Guncannons.)
Lt. Agar's Mudrock Gundam:
Obtain an S in any of the Federation Missions. (Note for his complete Mudrock, obtain an S in any Federation Mission again.)
Obtain an "S" rank in 4 out of the 6 Federation missions.
Different Mobilesuits, and weapons.
Sandra's Dom Trophen and Beam rifle:
Obtain 50 kills using Lt. Sandra.
Kai in Guntank:
Obtain 50 kills using Kai's Guncannon.
Hyato in Guncannon:
Obtain 50 kills using Hyato's Guntank
Nikki's GM Commander:
Obtain 50 kills using Nikki's GM team.
Lt. Fran's Guntank Mass Production Type:
Obtain 50 kills using Lt. Fran's GM S-Armor
Lt. Sandra's Guntank Mass Production Type:
Obtain 50 kills using San
Unlock Simulation "Hail Zeon!"
Obtain all "S" in all missions. This will give you all characters in all simulations missions. You can even use Feddie MS against in the regular Zeon Simulations.
Get an S rank on Simulation 15 to get LC Schmitzer.
Get an "S" on the Federation Simulation mission: Burning Sands, and after you do that you should be able to play as Kai and his Guncannon, and as Hyato and his Guntank.
Basically, sometimes in a mission you will encounter a group of GM's or something a little too risky to take on with your team. Even though you are undetected, a blitz then and there may cost at least a single MS in your team to be destroyed. While on the other hand, taking out the forces before you may either end the mission, or provide more rank points, and anyhow, make the mission that much easier if they are gone.
If there's not too much risk involved, do this. Take any other team that's not really doing anything major, best if you could free up all teams. Have them catch up with your squadron, as many teams as you can get. Before they get there, make sure you DISABLE their attack in the menu. This and setting their advance on AVOID>>> will work best, and the MS should not wander off and blitz the enemy themselves. When you have the team or teams, caught up with the original team, set them all to NOT MOVE, and DISABLE ALL of their ATTACKS. Then while they are frozen solid like rocks, set their ADVANCE to ATTACK>>>. Before this though, keep in mind the type of Enemy Units, and the MS types you have and their weapons. Let's say it's the first mission in round 2. Have 3 teams reach the point near the control tower, but dont alert the G GM's there or things can get ugly. Let's say you have Rohers 3 Gouf Custom team with 90MM Sniper Rifles, Austins Bazooka carrying Zaku I's, and Hepners Zaku II G's with 90MM machine guns. Since the machine gunners are the weaker of weapons, you should organize them as last in line to attack, placing them at the end of the three teams. Bazooka guys can hit, at this range, the GM's for one shot kills before the GM's machine guns even have effective range, and therefore they should be put in the middle, or right side, and to the left of them, Roher's snipers beside them, or in front. That would look something like this...
Hepner being the bottom, Roher top left, and Austin top right. Or you can line them up top to bottom in a row.
While they are set to not move, and not to attack, set their advances on ATTACK>>>, like i mentioned before. Then, starting off with the lead team, that being Roher's blue team, systematically go the the menu of blue, red, then green, setting the teams to attack and move. If done correctly, a full scale assault will ensue, regardless of the routes already set on the map. After the battle, you will have to re-assign the troops so they can continue the original plan.
Now that forest control tower setup was an example. That scenario can easily be done with Roher's Gouf Custom alone, with radar cloak and sonar turned on. He could sneak out just out of the G GM's range and snipe them all, so use the basic method on other missions where routes may interuppt your strategy. (therefore disrupting the routes to force an attack, then continuing the original route procedure). This also works great if you already set Go Codes and you need to move the teams away from them to help somewhere else on the map. It's time consuming, but it's effective, and may save your life and also earn you that S rank.
These are only that which I have personally seen unlocked, there are many more rumored, but I have yet to confirm them.
*For most of these you'll need to have beaten the mission mode at least once.
Unlocking EFF Shield(like that of Gundam and GM's)-After completing the game, successfuly complete practice mission 1 with Austin's Zaku I
Unlocking Lt. Commander Garret Schmitzer with his MS-05S Zaku I custom-Simply Complete the mission mode once
Unlocking Char Aznable and his MS-06S "Red Comet" Zaku II Custom-On simulator mission I "Practice" receive an A or an S ranking
For Garma Zabi and his MS-06Fs custom-Receive an A or an S in simulator 2-Practice II
For Ranba Ral and his YMS-07b Gouf-Complete the entire mission mode a second time up through at least mission 8
For Gaea of the black Tri-Stars and his MS-09 Dom-Complete up throug mission 12 a second time
For Agar in his guncannon completecomplete mission 10"Attack jaburo" a second time
In addition, complete simulations27 through 31 with an A or S to receive Agar;s RX-78-4 Gundam madrock
For Sayla Mass in the RX-78-2 gundam, receive a C or less in three seperate EFF simulation missions
For Amuro Rei, in the RX-78-2 receive an S in all EFF missions
I know Kai and hayato are selectable, but am unaware of precisely how to achieve them, if all of the afformentioned is completed, you should receive tham as well.