In both Endless Ocean and Endless Ocean 2, the player is able to befriend dolphins in order to get help from them in multiple tasks, like discovering treasures, and helping the player find creatures under zoom-mode spots.
In the first game, the player can choose only three out of eight available species (because the Narwhal, Spectacled Porpoise, and Amazon River Dolphin do not appear in the first game) to have as partners. The first partner the player can obtain is a Bottlenose Dolphin that had somehow become separated from its pod and became confused, because its echolocation was befuddled by the shallow waters of the Lagoon and Coral Forest. The player helps get it to deeper waters and thus gains its trust, meaning that the player will always have at least one dolphin partner all throughout the game.
To befriend a dolphin, the player must simply find the dolphin they want to befriend (any individual of the species, anyway), and learn as much about it as they can (unlocking each of the three information slots) and then continue to behave as if they are still learning about it; that is, continue to feed it, pet it, and use the sea whistle on it as much as possible. (There is no outward indication of when the player has interacted with the dolphin enough to befriend it, so lucky guesswork plays a minor role in this process.) Then, when they return to the boat, Catherine will note that a dolphin has come to the back deck of the boat to see the player, at which point the player can choose if they want to keep the dolphin, and, if they do indeed keep the dolphin as their partner, a name for their new friend.
The dolphins in the first game can also learn tricks, but not as specific or as easy to train as they are in the second game. They player can train these tricks by using the Sea Whistle off of the back deck of the Gabbiano, which happens automatically when the player selects the stairs that lead to the gangplank.
Endless Ocean: Blue World
In Endless Ocean: Blue World, the player can befriend a total of eleven species. However, unlike in the first game, the player cannot befriend just any specimen, and instead must follow specific steps in order to befriend certain individuals from each species, every one of which has certain defining morphological aspects:
- A Short-Beaked Common Dolphin, with a golden tail. This is the first dolphin the player befriends, as it is already an ally of L&L Diving Service by the time the player comes along.
- A lost, distressed Spectacled Porpoise. This is the second dolphin the player befriends in the regular story, found in the Iceberg Cavern. It becomes the player's partner after they calm it down and help it find a way out of the Cavern.
- A Bottlenose Dolphin with albinism. After rescuing it from a Tiger Shark, it becomes the player's ally.
- A Narwhal with a tusk that displays an odd scarlet colouration. It is a nervous creature, so the player must let Oceana comfort it before it will become their partner.
- A strangely verbose Beluga Whale with flower markings. After the player "sings back" to it with the Sea Whistle, it will become their friend.
- A completely pink Amazon River Dolphin. It seems shy of the player character at first, but after they let it interact with the Red-Tusked Narwhal, it decides to join the team.
- A rather hungry Risso' Dolphin. It will be befriended after feeding it squid meat and the seafood set that can be bought from Nancy.
- A Pacific White-Sided Dolphin with a unique fin. It is easily scared, but after some persistence on the player's part to comfort it, it eventually decides that they pose no threat and comes along with them.
- A False Killer Whale with an X shaped scar on its head. It has been hurt by a ship and is easily frightened. The player has to chase it around within Valka Castle for a little while before it eventually decides to see whether or not they're actually a threat - and decides that it will become their partner.
- A small Orca with a distinct saddle patch behind its dorsal fin. Hayako has seen this Orca before and she knows that it loves playing with floating toys. After the player buys a float toy from Nancy and offers it to the Orca, it will gladly come along with them.
- A Commerson's Dolphin with mysterious markings on its body. It seems to be some kind of emissary of the goddess depicted by the Pacifica Treasure, as the player must give offerings to the statue before it will go with them. (All of these offerings can be bought from Nancy.)
Tricks (Endless Ocean 2)
All dolph partners will learn six tricks, and some can be upgraded into better and more complex tricks after leveling up. To train these tricks, the player can call their dolphin partner to them using the Dolphin Bell on Nineball Island.
Level One Tricks
All dolphins learn the same tricks at level one:
- Jump/flip - The dolphin jumps out of the water.
- Sing - The dolphin "sings" a song.
- Backflip - The dolphin does a small backflip out of the water.
- Wave - The dolphin positions itself horizontally in the water and waves its front fins in a similar manner to the human greeting.
- Spin Jump - The dolphin does a small vertical jump with a little bit of spin.
- Tail Walk - The dolphin propels itself across the surface of the water using its tail fin.
Level Two Tricks
All dolphins learn the same tricks at level two:
- Advanced Jump/Flip - The dolphin does a larger jump, while flipping faster.
- Sing - The dolphin sings a different song than the level one song, and longer.
- Advanced Backflip - The dolphin performs a bigger backflip, while flipping faster.
- Wave - Nothing changes with the wave trick, except a slight increase in duration.
- High Spin Jump - The dolphin does a higher spin jump, while spinning faster.
- Tail Walk - Nothing changes with the tail walk, except a slight increase in duration.
Level Three Tricks
Some tricks may be replaced by a special trick. These are the defaults:
- Super Jump/Flip - The dolphin does an even larger jump, while flipping even faster.
- Sing - The dolphin sings a different song.
- Super Backflip - The dolphin performs an even larger backflip, while flipping even faster.
- Wave - Nothing changes but for a slight duration increase.
- Corkscrew - Your dolphin performs an even higher spin jump, while spinning rapidly.
- Tail Walk - Nothing changes but for a slight duration increase.