This one will bring back fond memories for all you Dragoons out there. If you're any kind of Final Fantasy fan then you'll know who Kain is, for those who don't, here's a brief explanation:

Kain is a Dragoon Knight that teamed up with Cecil in Final Fantasy II(U.S.), he has the same jump ability Freya has and to my knowledge pioneered the technique.

Now onto the nitty gritty. You wanna find Kain in the game? And I don't just mean his lance, I mean FIND him. Well it isn't too hard actually, all you need is Freya and Kain's Lance that you find in Memoria near the entrance. Once you have the necessary ingredients, level up in Memoria till around level 89-97, but careful to avoid levels 90 and 95 as you'll see later.

Now, when you're ready, make it all the way to the Crystal World and fight your way through till you get to the portal that leads you to the fight with Kuja. Stop and go to your item menu and arrange you first 5 items like this:

Once done, enter the portal and you'll see you aren't gonna fight Kuja just yet, you have taken a detour into a scarcely lit chamber. Walk straight till you run into an alter with the remains of some body. When you examine it a dialog talking about how this corpse was once a legendary Dragoon and was stripped of all his most powerful armor and his life after a deadly battle to save his wife and son. It then asks you to find and return these items to him so that he may rest in peace, this is when you use the items listed above (in order) starting with Kain's Lance. When the last item is placed on the alter you'll immediately go into battle complete with the piano version of Final Fantasy's main theme. You'll soon see that all the items you placed were in fact artifacts to resurrect Kain himself! But something is wrong, he's not a good guy anymore and seems to be a bit cranky from his eons of sleep. He blames you guys for cursing him into eternal unrest and sets forth and attacks with lightning speed and brutality.

This is a very tough boss fight if you're the kind that likes to go and just attack with everything you have, not so is the case here. Kain has 9,999 HP but thanks to you putting all those armor pieces he has regained all his Dragoon armor and can withstand a Grand Lethal Dyne attack and only take from 15 to 24 points damage.

But fear not, most of the time he will use his awesome Jump attack and be in the air for at least two turns. That's actually the way the battle first starts, he jumps in the air which gives you little time to cast protect and regen(if you don't have it auto'd already). But when Kain finally lands his deadly attack get ready to cast Curaga cause he'll drain all your characters down to at least 433HP! Other than that, all you have to worry about is his normal attack and his Drain Spear which he use every now and then to drain some HP from your members. Oh, and don't forget Level 5 Death!!! That's why I said no level 90 or 95!
Now if some of you were smart enough to have Dagger in your party, summon Bahamut and even though he only hits for 30-50 HP, you can see Kain has a significant reaction to him, he falls to his knees as if he was weakend or terrified by the King of Dragons. Now here's where some lineage comes in. You'll remember from Final Fantasy two, after Kain and Cecil set out on their journey out of Baron castle the prophecy pops up to intro the game. It goes something like this:

One born of a Dragon,
bearing darkness and light
shall rise to the heavens
over the still land.
Bathing the moon in eternal
Light, he brings a promise
to Mother Earth with
bounty and grace.

That was not about Cecil as most think, it was indeed intended for Kain as HE was born of a dragon and HE rises to the heavens. And HE bathed the moon with eternal light(with his Holy Lance).

So that means that Kain is in fact Bahamut's son, born from Mother Earth. So, what does this mean for you? It means that when Kain is stunned after the Mega Flare, ATTACK! You can kill him in one shot with Grand Lethal if you are in trance mode! If not, keep repeating this pattern and you will eventually win this historic battle.

Afterwards, Kain is "bathed" in light and explains the story of his demise and the curse that was put upon him to never ascend to heaven when he was slain. It seems Kain was up here long ago before Zidane and everyone else were born. He had been searching Memoria for the killer of wife and son, and when he finally found him, it turned out to be Necron himself, the final boss! So Kain lost, his Lance fell from the heavens to where you found it earlier, and his armor stripped from him and he slowly decayed but his spirit never went to heaven, poor guy.

Now that you have beaten him with his armor and weaponry on him, he can finally ascend to the heavens and rest in peace! Now what do you get for all this? Well, since he won't be needing his armor and Lance in heaven he decides to give them to you! Well…sort of, he gives them to Freya! You get:

Kain's Lance - Supercharged with his spirit power (Attack stat is now 277!!!!!)

Dragoon Helm - Absorbs Elemental spells

Dragoon Armlet - Absorbs Holy

Dragoon Armor - Casts Regen and nullifies DoomsDay and counter attacks with Ultima

"Anti"-Matter - Freya is rendered into atoms and her evade is bumped up to 99%, and her Jump ability is now enhanced by letting her shower multiple opponents with as many lances as she has HP, so 9999 HP equals 9999 lances thrown at the enemy.

Well, there's another one in the books for ya! I bet Freya fans are happy about this little secret, but remember, only a myth...