Brett is Jason's brother, and as it is usual with brothers and sisters, they sometimes quarrel, but then they quickly make it up again. Brett's primary intention is to have fun, and he's always looking forward especially to visits on Neri's island.
He's doing everything he can to keep Neri's existence secret, even if he has to risk a lot for that. Every time the kids strike against UBRI or PRAXIS, he's surely with them.
Charley is a humpback whale and a friend of Neri. This is possible because Neri is—due to her extraterrestrial abilities—able to communicate with him. In a test conducted by Dianne they find out that Neri reacts on frequencies that are as high as 60000 Hz and they presume that this is the reason for her special abilities. But the true reason is that Neri has the 'gift'.
Neri becomes acquainted with Charley when she is a little girl playing with a kite. She falls off a cliff into the sea. She screams for help, but her father does not hear her—in contrast to Charley.
In the adventures of the ORCA kids Charley often helps in risky situations. He frees Neri who gets captured in an attempt to help Dr. Hellegren on the rescue boat, because the doctor only has his scientific curiosity on his mind. He also calls Neri to help Mera when she fell off a cliff, when Jason got buried under a part of ORCA city's construction and finally when Kal steals the synchronium pieces.
On the other hand he can count on his friends at ORCA and on Neri and Mera when he is in danger. The kids help him when he is captured by UBRI and when he runs ashore on an island caused by UBRI's blastings.
Furthermore he plays a big role in the success of the ocean people's missions, because he is the one who can activate the synchronium, and he carries in his blood the antidote against the virus that Malakat used to contaminate the oceans of his homeworld and Shersheba those of the Earth.
Commander Lucas is the station's commander during the first season. He doesn't like children very much, except for his daughter Lee. When she's concerned, he immediatly goes to red alert. When Brett is missing in a storm, he cancels the rescue operation without compassion, because the safety of the well-trained rescue team is more important to him than that of a helpless ten-year-old boy. When he catches Jason with his daugther on the edge of kissing her, he goes almost mad.
When Brett gets suspicious that commander Lucas is after his mother, he plays a very nasty trick on him. He tells him a lot of things about her family, for example that she likes to eat garlic. So Lucas instructs the cook to put a special big portion of that in her meal ...
Lucas' relation to the kids doesn't get better until he owes his daughter's life to Damien.
Damien, one of the teenagers on ORCA, only appears during the first season. He is also known as 'the underwater fool', because he has no abilities worth mentioning except for the three chords he can play on his guitar.
He often retires to uninhabited parts of the station to play his music. There he is discovered by Lee once, and after having exchanged a few sentences, there is a seaquake and they find themselves locked up in the room which is slowly filling with water.
But Damien manages to pry the door open by playing on his guitar a chord of the appropriate frequency. Of course he has to demonstrate this to his friends later on. But this time not only the door opens, but the whole lighting of the room is sent to Nirwana.
Furthermore he is one of the kids to help Neri rescue Charley from UBRI's clutches.
Mrs. Byrne, the commander in the second season, is Mick's mother. She does everything for her son, but he won't take his chance and spends his time playing bad tricks on other kids on ORCA. When they complain with Mrs. Byrne, she won't believe them, because she thinks she had the smartest and most well-behaved son in the world. But in truth, her son dislikes her, for she doesn't really care about him and her only concern is her carreer. When she 'catches him in the act' of learning together with Vanessa, she thinks he'd spend his time with her and she gets mad at him. Finally, when they talk openly about it all, they settle their differences.
Commander Wellington is the station's commanding officer during the third season. At the same time he is a member of the tribunal and therefore a legitimate jugde. He is very aware of his responsibilities, and he reacts calmly and calculatingly, almost reservedly, in every situation. Should the children have done something wrong again, they get appropriate punishment from him.
Benny is the son of ORCA's chief medical officer, Dr. Malkovich. He had to come with him aboard ORCA and now he's attending school there. His parents press him, they want him to be a super pupil, no matter the cost. So he spends even his holidays learning for school, when all the other kids long since are enjoying their free time.
But after having met Cassandra and Brett, and after they made some effort convincing him, he changes his mind. At first, they play tricks at each other, later they take actions against UBRI together, to help Neri recover the Synchronium pieces. During these, he goes through fire and water for his friends several times, and he proves true courage. To protect Neri's secret, he also accepts the risk of lying, even against the commander.
The only thing Cassandra has on her mind in the beginning is to annoy Brett, who always retaliates in an appropriate manner. But sometimes she exaggerates her actions, and usually she will have to sweep the floor as a punishment then. But that doesn't matter for the young American girl, as she is—exactly like Brett—always looking for fun and adventures. These qualities they have in common quickly lead them to mutual understanding and they become good friends.
At first Cass thinks it's totally boring on ORCA. But when she gets to know Neri, she finally found an occupation: to help Neri in her exciting adventures. No one is able to hold her back in the search for the synchronium, and when it's up to her and her friends to protect Neri from PRAXIS, she really is unstoppable.
There is one thing Cassandra does not like at all: her big sister Morgan, the cadets' leader on ORCA. All the time Morgan tortures her with questions about what she was up to again, and she always finds an unpleasant job for her sister. Once when Cass was sentenced to clean the floor with Brett and Benny, Morgen sent the two boys away and had her sister do the whole work, until the floor was shining.
The two do not settle their differences until Morgan falls in a coma after being bitten by a stonefish. Cassandra sits besides her and holds her hand.
Dave Hartley is the first officer on ORCA. He is always friendly towards the kids and he teaches Winston how to swim and dive when he is in danger of being dismissed because he cannot swim. Mr. Hartley always stays calm and composed, even in risky situations. During the third season he trains the cadets, during the forth he only has a few appearances.
Brett thinks Dave would be quite suitable as a new husband for Dianne, and so he once writes a faked anonymous love letter to his mother. But the plan fails, as the letter gets in the hands of Sally-Anne who thinks Jason sent it.
Billy is a very young member of UBRI's staff, he's working as a spy there. He's commisioned with the preparation of Operation Jonas I. He's instructed to steal the recordings of whale songs from Dianne's and Winston's lab.
He successfully completes this mission. But when he's caught in the act in another spying mission, he confesses the kids that he in truth can't harm a fly.
| 1.5 | Young Neri (Leilani Krans) |
| 1.7 | Blunt (Tim Robertson) |
| 1.7 | Lambert (Neil Melville) |
| 1.8 | Valde (Tenley Gilmore) |
| 1.8 | Valerie (Emanuela Leoni) |
| 1.8 | Girl (Neri look-a-like) (Leila Koren) |
| 1.9 | Sam (Jermaine Herbohn) |
| 1.9 | Kevin (Lyndon Daly) |
| 1.9 | Matt (Rhys Hine) |
| 1.9 | Nick (Michael Richards) |
| 1.9 | Gift Shop Owner (Louise Benetto) |
| 1.9 | Shop Attendant (Megan Holloway) |
| 1.9 | Couple No. 1 (Bianca Griffin, Todd Pilkington) |
| 1.9 | Couple No. 2 (Lisa Thorpe, Philip Bartlett) |
| 1.9 | Cinema V/O (Abbe Holmes, Hamish Hughes) |
| 1.9 | Smithfield High (Andrew Duffin) |
| 1.9 | Corroboree Group (Frederick Duffin, William Duffin, Daniel Glutchen, Dennis Hunter, Gordon Hunter, Timothy Wason, Sam Wason) |
| 1.10 | Jan Slater (Helen Jones) |
| 1.10 | Medical Officer (Stephen Hall) |
| 1.10 | ORCA Nurse (Rebecca Strok) |
| 1.? | Celia (Doris Yoijname) |
| 2.? | Joshua (Eric Milliken) |
| 2.? | Sheve (Andrew Bibby) |
| 2.? | Patrick (Daniel Bowden) |
| 3.4 | Larissa |
| 3.11 | Moose |
| 3.11 | Mareika |
| 4.15 | Emma |
| 4.15 | Luke |