In Endless Ocean, 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 their pod and gotten confused, because their echolocation was befuddled by the shallow waters of the Lagoon and Coral Forest. The player helps get them to deeper waters and thus gains their trust, meaning that the player will always have at least one dolphin partner all throughout the game. After the player helps the bottlenose and and returns to the boat, Katherine will mention that dolphins are considered "holy messengers" in Pelago myth before giving the player the Sea Whistle.To befriend a dolphin, the player must simply find the dolphin they want to befriend (any individual of the species, anyway), 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 say: "Look at that! You ever seen that dolphin before? Maybe he's here to visit you. This is a special occasion. Why don't you go down the gangplank and see him?"
At that 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. If not, Kat will interpret the dolphin as saying "'Aw, that's OK! Come back to [location dolphin was found] if you want to see me.' ...Um... I mean... That's what I think it said." If the player interacts with a dolphin of the same species that they declined to keep as a partner, Kat will comment "I wonder if that was the one who visited us at the boat? I hope he comes back soon! He's a cute little guy!"
The dolphins in the first game can also learn tricks, but not as specific or as easy to teach 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.
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.
Some partners are gained simply by playing through the game, while others require the player to complete their respective parts of the Be the Best Trainer quest. The befriendable dolphins are unique amongst other members of their species through defining traits such as having strange markings, unusual coloration, odd behavior tics, or differing size.
The first partner the player can befriend is a short-beaked common dolphin with a golden tail. This dolphin is already an ally of L&L Diving Service by the time the player meets them, so befriending them is automatic.
The second partner the player can befriend is a spectacled porpoise with white markings. Befriending them is part of the main story, as the porpoise helps guide the player out of the Weddell Sea during a severe blizzard.
The third partner the player can befriend is a bottlenose dolphin with albinism. They can be found at Cross Rift in Ciceros Strait, being attacked by a tiger shark. After calming the shark, the player gains the trust of the dolphin.
A hungry Risso's dolphin appears in the Zahhab Region Depths, in the Northern Crevasse. He can be befriended during the Be the Best Trainer quest, by feeding him with seafood that can be purchased from Nancy's shop.
A white-finned pacific white-sided dolphin with strange white fins appears swimming above Deep Hole, with the pod of the same species that already lives there. They can be befriended during the Be the Best Trainer quest, by visiting them multiple times until they feel comfortable.
A smaller-than-usual orca with a scarred forehead, most likely a calf, appears in the Southeast Zahhab Region, past the Open Sea. They can be befriended near the end of the Be the Best Trainer quest, by letting them interact with Hayako and giving them a plastic toy that can be purchased from Nancy.
A white-marked Commerson's dolphin appears in the Cavern of the Gods, and can only be encountered after it is re-opened; as such, they will likely be the last dolphin the player befriends. They can be befriended as a part of the Be the Best Trainer quest, by offering "gifts" to the Queen of the Okeanos, all of which can be purchased from Nancy.
In Endless Ocean, the player can train their dolphin partners by heading to the gangplank at the back of the Gabbiano. They summon their partners using the Sea Whistle, as well as any potential new partners that might have decided to visit in the meantime.
When selecting tricks, the player can choose as many as they'd like in a row, as opposed to the limits imposed upon them in the second game. They can choose which tricks to train by selecting either the dolphin's head or one of the pectoral fins, and then poking, stroking, or grabbing it. Each of these six configurations chooses a different trick:
- Poke (Press A) - Spin Jump
- Stroke (Hold A and shake the Wii remote) - Front Flip
- Grab (Press A and B at the same time) - Backflip
- Poke - Tail Walk
- Stroke - Sing
- Grab - Handshake
Endless Ocean: Blue World
All dolphin 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.
Training dolphin partners is widely considered more streamlined and less confusing in the second game.
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.