EX modes are used to initiate EX Burst,which is a special attack.Consider it as Limit Break or Overdrive from FF games in Dissidia.CHanging to EX modes will make various effects,such as increased attack power or so on.
Here are the list of EX modes and EX Bursts.
Warrior of Light (FFI)
EX Mode - Changes to Knight Form
EX Burst - Oversoul [A powerful sword combo finisher]
(Press 6 directional button to increase the attack power)
Garland (FFI)
EX Mode - Changes his armour to white with some weird patterns.
EX Burst - Soul of Chaos [A powerful sword combo finisher bu changing Garland's sword into two]
(Must tap O button repeatedly to increase attack power)
Firion (FFII)
EX Mode - Changes his weapons to red.
EX Burst - Fervid Blazer [Firion strikes his enemy with every weapon in his arsenal]
(Input correct buttons to increase attack power within the time limit)
Emperor Mateus (FFII)
EX Mode - Changes to Emperor of Hell form
EX Burst - Absolute Rule [A Powerful explosion attack]
(Input correct buttons to increase attack power within the time limit)
Onion Knight (FFIII)
EX Mode - Changes to Ninja or Sage form (Can be chosen)
EX Burst - Ninjutsu (Ninja EX Mode), Great Magic (Sage EX Mode)
[O.K will use the Throw command in Ninjutsu mode,and will choose some magic and Meteor in Sage mode]
(Input correct commands to increase attack power within the time limit)
Cloud of Darkness (FFIII)
EX Mode - Changes to green coloured form of herself.
EX Burst - Super Wave Cannon [Cloud of Darkness will unleash a devastating cannon attack]
(Must tap O button repeatedly to increase attack power)
Cecil Harvey (FFIV)
EX Mode - Attacks using with his swords whether in Paladin mode or Dark Knight mode (His spear or staff is abandoned when using EX mode)
EX Burst - Double Phase [Cecil will switches between Dark Knight and Paladin modes to unleash a powerful finisher to his opponent)
(Input correct directional and attack buttons to increase the attack power)
Golbez (FFIV)
EX Mode - Fuses with a Shadow Dragon
EX Burst - Double Moons [Golbez will use the powerful W.Meteor attack]
(Input correct directional buttons to increase the attack power)
Bartz Klauser (FFV)
EX Mode - He will uses Brave Blade,three stars will appear above his head,and will use a red cape.
EX Burst - Rapid Fire Dual-Wield Spell Blades (Combines all of other FF heroes weapons into one to attack his opponent)
(Condition unknown)
Exdeath (FFV)
EX Mode - Tree form
EX Burst - The laws of the universe mean nothing! (EXdeath will use Neo Almagest or Grand Cross,depends on how long you hold the O button)
(Hold down the O button to increase attack power in the bar)
Terra Branford (FFVI)
EX Mode - Esper Form
EX Burst - Riot Blade [Terra unleashes a flurry of powerful crescent energy waves]
(Must press the O button simultaneously with the directional buttons to stop two energy meters.The more the meter is,the more powerful the attack)
Kefka Palazzo (FFVI)
EX Mode - God Form
EX Burst - Light of Judgment [Kefka will use a powerful light attack to destroy his opponents]
(Must input correct buttons to use Light of Judgement,if not,a weaker attack Forsaken will be used)
Cloud Strife (FFVII)
EX Mode - Changes his Buster Sword to Ultima Weapon
EX Burst - Omnislash [Cloud will unleashes powerful 14 slashes in a rapid succession]
(Must tap O button to increase the power bar to increase the attack power and the number of slashes)
Sephiroth (FFVII)
EX Mode - Reunion (Sephiroth will gain a black wing on his back.)
EX Burst - Super Nova [Sephiroth will unleash a super nova to destroy his enemies]
(Must tap O button to increase the power bar to increase the attack power)
Squall Leonhart (FFVIII)
EX Mode - Changes his Revolver to Lion Heart
EX Burst - Renzokuken [Squall will unleash a powerul combo,and will use Lion Heart attack as the finishing blow]
(Must time button presses with the flashing bars on a power bar,similar to FFVIII)
Ultimecia (FFVIII)
EX Mode - Fuses with Griever
EX Burst - Time Compression [Details unknown,but the finsihing blow for Ultimecia's EX burst will be End of Memories]
(Must time button presses with the flashing bars on a power bar,similar to FFVIII)
Zidane Tribal (FFIX)
EX Mode - Trance Form
EX Burst - Reverse Gaia [Zidane will unleashes powerful beams to attack his enemy]
(Details unknown)
Kuja (FFIX)
EX Mode - Trance Form
EX Burst - Last Requiem [Kuja unleashes mutiple explosions to destroy his enemy]
(Must tap O Button to increase the number of explosions)
Tidus (FFX)
EX Mode - Changes his Brotherhood to Caladbolg
EX Burst - Blitz Ace [Tidus unleashes a powerful combo,and kicks a blitzball to his enemy as the finishing blow)
(Must press the O button to stop the cursor in the middle of the energy bar to increase attack power,similar to FFX)
Jecht (FFX)
EX Mode - Final Aeon
EX Burst - Blitz King (Jecht kicks his opponent to the air,slashes him/her endlessly,and kicks a boulder to the opponent as the finishing blow)
(Must press the O button to stop the cursor in the middle of the energy bar to increase attack power,similar to FFX)
Shantoto (FFXI)
EX Mode - Black Mage Relic Armor
EX Burst - Magic Burst [Details Unknown)
(Details Unknown)
Gabranth (FFXII)
EX Mode - Full Armoured Judge Form
EX Burst - Exhibition (Gabranth will use Fulminating Darkness,Ruin Impendent, and Flame Purge to activate certain Concurrences,like Black Hole or Inferno)
(Commands must be inputed quickly to achieve higher Concurrences like Black Hole)
Here are some images of some EX Modes and EX Bursts.
Terra's Esper Form
Cloud's EX Burst,Omnislash.His Ultima Weapon is present.
Sephiroth's EX Burst,Super Nova.His EX Mode,Reunion (Black wing) is present.
Zidane's EX Mode,Trance form.
Kuja's EX Burst,Last Requiem.
Tidus's EX Burst.His Caladbolg is present.
Gabranth's EX Burst,Exhibition.He is using Ruin Impendent in thsi image.