Ketos (also called Ocean Master Ketos) is the guardian of the Undersea Grotto, fought at B8F (The proud, tall waves).
This huge, ancient whale is a faithful servant of the king of the Deep City. Even so, he is treated with great respect, by both the said king and his servant. Also, he is capable of telepathy. Although unwilling to fight the explorers, he does so, as its his duty as the guardian of the city. After his defeat, before dying, he shows respect toward the explorers, calling them great ones.
Ketos is a very capable fighter. Despite his size, this colossal, ancient whale can swim with great agility. He can use both water and ice attacks, and use a ancient song to make its enemies to fall in a deep sleep, being unable to evade his powerful attacks. Grand Berry can paralyze a party member, although it will only hit one party member.
Ketos will use Spout on the first turn and subsequent turns that are multiplies of 5. This increases his speed and evasiveness. Ketos will continue doing so until his HP has been brought to the red.
Between the turns, Ketos will use various attacks.
Freeze Ripple is a multi target ice attack that have a chance to leg bind the party members hit. This is important to note because another powerful ability, Ocean Rave, will only hit party members that have bind.
Grand Berry is a single target hard hitting attack that may paralyze.
Once Ketos' HP has been brought down to the red, he will start using Great Melody that can put party members to sleep. He will also stop using Spout and will use his attacks more randomly.
It is a good idea to use Metopon or Ad Nihilo to nullify Spout. It is also highly beneficial, though not necessary to have a monk with high level Unbind. This will prevent you from getting hit by Ocean Rave at all. Grand Berry usually targets front row so keep your weaker party members in the back. Just be prepared when Ketos starts using Grand Melody since this makes the fight potentially more unpredictable.
Spout - Self buff, increases evasiveness.
Freeze Ripple - Deals moderate ice damage to the whole party, may inflict leg bind.
Ocean Rave - Deals heavy Strike damage to party members with leg bind.
Grand Berry - Powerful single target Strike damage, may inflict Paralysis.
Great Melody - Hits the whole party, put party members to sleep.