I think that the Guardian Forces are a neat addition to the Final Fantasy series. You can raise them and give them new abilities to aid the character they're junctioned to. And they have unlimited uses. They have HP, so instead of using MP (which has been removed in this game) you can just summon them until they take too much damage and are KO'd. These are all the Guardian Forces along with the ones that require items to use in battle (secret ones). And most of these Guardian Forces attack all enemies.
Quetzelcoatl
How to Acquire:You can get Quetzelcoatl at the beginning of the game, just go to Squall's computer at his desk, then click on tutorial.
In Battle:Quetzelcoatl's element is lightning. Once summoned it lifts it's wings and flaps them until a HUGE lightning bolt crashes down on all enemies. (Lightning-elemental)
Shiva
How to Acquire:Same as Quetzelcoatl.
In Battle:Shiva hasn't changed much (except her looks) since she was in FFVII. She is still the goddess of ice and her power is imense. Shiva is shown asleep in an ice barrier, but then her eyes quickly open and she explodes from inside the barrier. Then with a wave of her hands she covers the enemies in ice (cool!). While Shiva isn't the strongest GF, she can kill off weaker enemies with ease. (Ice-elemental)
Ifrit
How to Acquire:You must challenge Ifrit to a fight to get him to join your cause. I hope you leveled up some and make sure to junction Shiva to someone.
In Battle:Ifrit puts his feet on the ground only to jump back up and throw a humungous fireball (meteorite?) at all enemies. (Fire-elemental)
Siren
How to Acquire:You must draw Siren from Elvorlet at the top of the Communications Tower (don't forget because Siren has very useful abilities like Treatment and Tool Refine!!!).
In Battle:A small wave crashes down on the enemies and then the screen is filled with water. In the middle on a rock is a beatiful winged woman. Siren's wings float elengantly in the air as she plays a song on her harp that damages all enemies and may silence them. (non-elemental + Silence)
Brothers
How to Acquire:After you've seen the missing student's ID#, in the Tomb of the Undead King, continue on into the temple until you see a weird looking Minotaur. Go up to it and you'll engage in combat. It runs after being defeated so you must solve the puzzle (pull a couple of switches) to create a bridge to the tomb where you will fight the Minotaur and his brother, Sacred. They join after being defeated.
In Battle:Minotaur lifts the ground from up under the enemies and throws them in the air. The Brothers then play a game of rock, paper, scissors and Minotaur loses. Sacred then throws Minotaur into the air, as tears gush down his face as he hurdles into the enemies. (Earth-elemental)
Diablos
How to Acquire:After you finish your SeeD exam Cid will give you a Magical Lantern. *IMPORTANT* save before using this. Once used you'll be carried to another demension where you'll fight Diablos. This is not an easy fight! After being defeated Diablos will join.
In Battle:Bats fill the screen as a black hole appears. Diablos emerges and plunges his hand into the black hole. He then brings his hand down and throws the black hole (now a ball) at the enemy. (non-elemental)
Carbuncle
How to Acquire:You have to draw it from one of the two lizards (bosses) at the top of the the building (Edea's chamber) in Deling City.
In Battle:A hole forms in the ground and out pops Carbuncle. He jumps into the air and the ruby gem on his forehead glows as beams of light fall on each party member (non-elemental + Reflect cast on all party members)
Leviathan
How to Acquire:You have to draw it when you are fighting NORG.
In Battle:A vortex appears as Leviathan slithers out of it like a serpent (which it is). It looks at the enemy and screams and then jumps into the air as a mountian/waterfall is baing formed. It screams as it lands and a BIG waterfall rushes down on the enemy. (Water-elemental)
Pandemona
How to Acquire:Draw it from Fujin when you battle her and Raijin in the town of Balamb.
In Battle:Pandemona appears and uses a tornado to suck up all of the enemies and then spit them back out. (Wind-elemental)
Cerberus
How to Acquire:You have to fight and beat Cerberus (at Galbadia Garden, at a certain piont in game) in order to gain its help.
In Battle:In a dark and dreary place, big black gates open and out runs Cerberus. It opens its mouth and light shines as three rays fly out and hit your party. Then Double and Triple are cast on you. (non-elemental + Double and Triple cast on all party members)
Alexander
How to Acquire:Alexander must be drawn during your second battle with Edea.
In Battle:Water rises to the top of a nearby mountain as Alexander approaches. Alexander stomps his feet on the sandy ground and opens up his cannons and shoots out heavenly rays of light. The light travels to your enemy at blows them up. (Holy-elemental)
Doomtrain
How to Acquire:To obtain Doomtrain you must first get the Solomon Ring from in front of the HUGE statue at Tear's Point. Then you must get 6 Remedy+, 6 Malboro Tentacles, and 6 Steel Pipes. Once you have all these items go to the items screen and use the Solomon Ring and it'll join you.
In Battle:A long railroad track made of fire appears and then Doomtrain comes running through. After a few seconds you see your enemies standing at the tracks and then...BOOM!, Doomtrain flattens em' all! (Inflicts Poison, Vit 0, Darkness, Silence, Berserk, Sleep, Doom, Curse, Petrify, Confuse on all enemies)
Bahamut
How to Acquire:Once you have the Ragnarok you can enter the Deep Research Center (middle of ocean, inthe lower-left corner of world map). Once there you'll need to walk once the the green light shining is at its minimum, or else you'll be attacked (even with "Enc-None" junctioned). Once you are at the big tube you will be asked 3 questions. You don't have a choice for the first one, but choose "Never" for the second one, and then go all the way down to a secret third answer for the third question. After the first two question are answered you'll have to fight Ruby Dragons. And after you answer the third question correctly you'll face Bahamut. Defeat him and he'll join.
In Battle:You'll see thousands of clouds with a dark figure flying through them. The camera comes closer and we see Bahamut's wings spread out madly flying towards.....you're enemies. Once there his wings spread even wider and he opens his mouth to shoot out his Mega Flare attack.(non-elemental)
Tonberry
How to Acquire:Once you've gotten Odin, return to the Centra Ruins. Once there you have to defeat 20 Tonberrys (you have to "defeat" them, commands like "Card" and "Devour" don't count).
In Battle:
Tonberry appears and attacks with its chef knife to one enemy at random. (non-elemental)
Cactuar
How to Acquire:Once you have Ragnarok, fly to Cactuar Island, which is near the eastern Centra continent. When you're there you may notice a big cactus-looking thing popping up and down every so often. You must run into it to engage in combat, defeat it to get the GF, Cactuar.
In Battle:Cactuar jumps into the air and begins to shine, it then throws thousands of needles at your enemies. Its strength depends on its level (Lv 50=5,000 damage, Lv 80=8,000 damage, etc.) (non-elemental)
Eden
How to Acquire:After Bahamut has joined enter the Ragnarok and go to the room near the elevator and your party members will say things about the Deep Research Center. After that go back inside the Deep Research Center and you'll see a hole where the tube once was. Crawl down and then you'll have to complete a puzzle (*hint* at the end of the puzzle there must be 10 RSP remaining). Once completed doors to the lower remains of the Deep Research Center will be opened. Travel farther down and then you'll be attacked bu Ultima Weapon. Defeat him and Eden joins. But this is not an easy fight to win, but Eden is the strongest GF, so it's worth it. Someone with the command abilities Recover and Revive are useful as well as Quistis (cast Aura on her). But anyone with a high enough level will do (make sure all your party members have 9,999 HP!!!)
In Battle:Eden slowly comes to the ground from the heavens. As she lands she sucks the enemy into the air and examines it. She then shoots a powerful beam that plummets the enemy into a black hole. It lasts a while so you can easily boost her to 250 points. (non-elemental)
As the title suggests, these Guardian Forces require certain items to be used in battle. However, if you want to summon them more than once you must have more than one of that item (except for Mog and Angelo).
Chocobo
How to Acquire:You must first go to a Chocobo Forest and catch a Chocobo. After you catch it the Chocobo boy will give you a Chicobo (baby Chocobo). Once this happens you can find items called "Gyshal Greens" at various places in the game. When used in battle your Chicobo will appear.
In Battle:Chicobo runs up to the enemy and rapidly kicks them for minimal damage.(non-elemental)
Mog
How to Acquire:You must have a Pocket Station (sadly, only available in Japan) to get Mog's Amulet. Once you acquire this item you can exchange it to your FF8 game and teach a GF this ability. But you can use a Gameshark...
In Battle:Mog floats around and heals all junctioned Guardian Forces. This restores a great amount so its very useful during boss encounters. (non-elemental + heal all GF)
Moomba
How to Acquire:You have to have an item called "Friendship" to summon Moomba in battle. You can only get this item with a Pocket Station.
In Battle:Moomba rushes towards your enemies and rapidly punches them. Moomba actually dishes a good bit of damage.
Pheonix
How to Acquire:You have to find an item called "Pheonix Pinion" (1st one is a gift from Shumi Tribe Elder). Even after you use the item, Pheonix will continue to appear randomly during battles (like Odin) when one of your party members faint.
In Battle:Pheonix flaps its fiery wings and casts a strong fire spell on enemies while also reviving all fainted members.
Angelo
How to Acquire:Okay, okay! So this isn't really an item based GF (or GF for that matter). Angelo is Rinoa's dog and a part of her Limit Breaks. All you need to do is have Rinoa in your party and lower her HP (or cast Aura) and you can use Angelo (sorta).
In Battle:Angelo has many attacks. Some of the more useful ones are "Limited Moon"( makes party invincible) and "Wishing Star" (major, multiple damage to all enemies).
Odin
How to Acquire:I know, this isn't an item required GF either. To get Odin you must go to the Centra Ruins. Once there a timer will appear. You must find Odin bafore the time is over or else... When you find him you'll enter battle, defeat him within the tme left or Odin will automatically kill your whole party. (Odin can't be junctioned, but randiomly appears in the beginning of battle to instantly kill all enemies).
In Battle:Odin rides towards your enemies on his horce. As lightning strikes, he pulls out his sword and cuts out all enemies in half.
Gilgamesh
How to Acquire:During a certian event in the game Odin will be changed into Gilgamesh...
In Battle:Gilgamesh appears and sellects one of the three swords surrounding him. The Exaliper sword does nothing, the Excaliber sword damages all enemies, wil the Zantetsuken sword instantly kills all enemies.