It called when two attacks cancel each other out

The trick to doing just guard

To do the just guard, you have to press and let go of the guard button (for a total press of four frames or less) while your opponent is mid swing. You have to have already let go of the button before the attack connects to activate just guard. You have to let go of guard roughly 0.25 seconds before the hit connects.

The best way I've found to practice

Go to training and select a fairly slow character like Nightmare or Astaroth as your opponent. Set the CPU to counter after attack. The computer will automatically attack after each time you do. It's pretty easy to nail down the exact timing when you initiate it with your own attack. Attack, just guard, attack, just guard. After a while you'll be getting it on every try.

Practical timing in a live match

The three scenarios I've found the easiest to time against real opponents are:

1. Immediately after a guard. Many less experienced players will counterattack as soon as they're done blocking. If you practice using the method described above, it should be pretty easy to time when that counterattack is coming.

2. The last hit of an obvious combo. Most characters in the game have their trademark combos that a large portion of players use (usually B, B). Getting the timing between these two hits down is relatively easy. Guard the first hit and use just guard on the second. The timing should come naturally after some practice.

3. Unblockables. Probably the hardest of these three to time, but totally worth it if the people you play against favor using these. Unblockables are generally slow, so you have a second or two to prepare yourself and get the timing just right.

I found a list of the characters and the unlock conditions: HERE

Notice some have multiple unlock methods, some of which are easier than others.

Also here is a list of them all for quick reference:

• Special Character 1

  • Algol
  Method 1: After completing Story Mode, the Legendary Souls Mode will be unlocked. Defeat him in Legendary Souls Mode and then successfully complete the mode to unlock Algol.
  Method 2: Defeat him in Quick Battle as the opponents Teramos (C2-C4 Rank) or Minami Naikos (A2 Rank) to unlock Algol.
  Method 3: Defeat him in Arcade Mode with any character.
  Method 4: You can also reach Player Level 31 to unlock Algol.

• Special Character 2

  • Alpha Patroklos
  Successfully complete Story Mode with any character.

• Special Character 3

  • Dampierre
  Method 1: Purchase him before release as a Best Buy pre-order in America.
  Method 2: Buy him during launch as part of the Collector's Edition in Europe.
  Method 3: Other than that you'll have to wait to get Lord Geo Dampierre as future character DLC.

• Special Character 4

  • Edge Master
  Successfully complete Story Mode with any character.

• Special Character 5

  • Elysium
  Successfully complete Story Mode with any character.

• Special Character 6

  • Kilik
  Method 1: Defeat him in Legendary Souls Mode and then successfully complete the mode.
  Method 2: Defeat him in Arcade Mode with any character, but make sure to choose the Asia route.
  Method 3: You can also reach Player Level 11 to unlock Kilik. Method 4: You can defeat him in Quick Battle as the opponent “Chaos” with the title “Red Lotus” (A2 Rank) to unlock Kilik. – Two gamers have confirmed that before the match Kilik was locked and after beating “Chaos” he was unlocked.

• Special Character 7

  • Pyrrha Omega
  Successfully complete Story Mode with any character.

There is also several unlockables for Character Creation and Stages which can be unlocked in Story Mode and Quick Match.