Manaka and Hikari are childhood friends, she cares greatly for him and it is one of the few things we're introduced to at the start of the anime is that Hikari is in love with Manaka. Manaka is the only one who can tell if Hikari is hiding something. In part 1, Manaka admitted Hikari scared her and after words with Chisaki, she began to change herself, becoming braver and then realizing Hikari was only trying to protect her. As Hikari is always worried about her and does his best to make sure she is never upset or hurt. It is revealed that Manaka and Hikari share a special bond as she is always by his side as he is always by her side, for when Hikari crushes his toe, Manaka jumps into action and races to him before anyone else, she then processed to help Hikari with everything possible as she states that she want to protect him as he does her.
Manaka is the only one that seems to notice certain things about Hikari. She notices he is restless on the surface and has trouble with surface people, she helps him over come this in many different ways as she decides to do her best to help him. After this, she was the one who was always at his side. Hikari is embarrassed when he finds out that Manaka trusts him very much as she shows him a secret that she had become afraid of. She reveals the fish-head on her knee to which the boy tells her it is pretty cute but if it upsets her so much, he'll help cover it up. Nearing the end of part 1, Hikari in desperation hugs Manaka tightly, he is hurt when she pushes him away in confusion but feels relief when she later apologizes and admits it surprised her. Hikari almost loses himself in his feelings later on when he catches Manaka cooking, he wants to kiss her and tries desperately to block the thoughts and eventually ruffled her hair and ran away. Hikari then is pushed to admit his feelings to her, which once again surprises her and out of fear she runs away from him. After thinking deeply about it, Hikari decides to confess to her properly and goes out to look for Manaka and does so, Manaka listens eagerly. The next day, she and Chisaki are walking back home when Hikari spots Manaka and calls out to her, waving a massive flag around. She tells him later that after the ceremony, she has something very important to tell him as she has realized that her feelings for him has changed. Right before it ends, both Manaka and Hikari go into hibernation and their last thought is of eachother as they saw eachother before falling into a deep sleeping.
In part 2. Manaka is always on his mind. He wishes to find her quickly. When Kaname wakes up, the two discusses Manaka and how Hikari won't change himself anymore for her sake as he is still deeply in love with her and his feelings are still growing strong. In episode 18, Hikari finally finds Manaka and panics when he notices something very odd. He and Kaname tries desperately to save her and take her to the surface. When the two believes the sea-god is upset that he is taking her back, he simply says he doesn't care. He tries effortless to awake Manaka but fails, he even gets sick over it. He is so sad and hurting that he accidently mistakes Miuna for Manaka when Miuna has her hair down, he is greatly disappointed that Miuna isn't Manaka. He also has a nightmare about Manaka not wanting to come back to him. And because he had changed so much, Manaka sup doesn't know him, but when he and Miuna gets into a fight about Hikari kissing Manaka, Hikari acts like himself. He acts like his old self, the person Manaka knows, the person Manaka's sup feels. And so. Hikari is surprised when Manaka wakes up and scolds him for yelling at a woman. Hikari is overwhelmed with happiness at seeing her awake.
As the series progresses, Hikari began to notice odd things. He noticed that Manaka doesn't recall some important memories the group had spend together such as being cursed by Lord Uroko and lying about the fish heard, Manaka does not recall lying about still having the fish head when it left her after she had accepted it. Hikari also noticed how her eyes would glass over whenever they would talk about Manaka and Hikari together, eventually Hikari and Manaka walk around the middle school together, talking about the past. The two come to the old swimming pool and Hikari brings up the moment when he had crushed his toe and Manaka was the first to aid him, thus results in Manaka admitting certain moments in her memory are blank. That she doesn't remember things. Hikari desperately asks her questions about the time they had been together, not just with him alone but with the group, each memory he brings up results in Manaka shaking her head and saying she doesn't remember. He finally questions that faithful night all those years ago when he had confessed to her. Hikari his heart broken when Manaka finally and sadly says she doesn't remember. Manaka admits she had tried to remember that night, that she feels something big had happen during that night but each time her head would hurt and she would feel empty. Thus results in Hikari panicking and running away to cover up his broken expression. For the first time waking up, Hikari cried.
In the process of running blindly, his view filled with tears, Hikari stumbles upon Lord Uroko. He finds the fish man chilling out within an old shrine. Finding said man, he questions about the freezing of the surface and sea, only to be answered that everything had speed up because Manaka had be removed from her sleep place. That Manaka was used as the sacrifice to the sea god with out realizing. Manaka had done the only thing she could think of to save Akari and offered herself instead unaware that she was saying that she would become the sacrifice instead. Thus makes Hikari angry and he tries to punch Lord Uroko, only to have the man jump out the way. He eventually finds out that the thing the sea god took from Manaka wasn't her Ena. When finding out that she was the sacrifice Manaka refused to become the sacrifice and ripped off her Ena and Hikari unaware of this, freed the Ena into the sea. Lord Uroko reveled that Manaka's Ena was in fact her heart, the part of her heart is able to love. He reveled that to be freed, the sea god took Manaka's will to love from her. Hikari is shocked as he realizes his feelings will never reach Manaka, within this distress, he runs as far as he could from Lord Uroko. Letting out his frustration upon a snow covered wall, he is hurt and angry that everything he had done to make Manaka aware of his feeling were wasted. Finally feelings he has to see her, he runs back to Manaka and discovers her eyes were blank of love, she smiles at him and welcomes him back lovingly, but sadly there was real feeling behind it.
Hikari becomes determine to get Manaka to remember everything. The boy tries everything he could think off. Hikari spends every waken moment with her and finds a sea slug stone which shines brightly. This makes him think that whoever confessed to slug must be happy, because the stone is shiny and clear meaning their feelings will grow stronger and bloom. Manaka admits that she remembers telling a red bellied slug something important but couldn't remember what or what her stone was. Hikari dives into the water and gives the stone, saying it should belong to her. Wearing the stone around her neck, pressed against thee spot where her heart is, the stone absorbs all the feelings Manaka should be feeling. Thanks to the stone, it is soon revealed that Manaka confessed her true feelings to the red bellied slug. Admitting she had always been in love with Hikari but was scared. The stone eventually erupts with Manaka's feelings, screaming with Manaka's voice that she loves Hikari. Tsumugu revels that he had kept an important promise to Manaka, that the two had talked upon his boat and she had admitted to him that she had always been in love with Hikari but was worried because Chisaki liked Hikari too at the time. Manaka admits Hikari is like her beloved sea.
Hikari is excited upon finding this out, especially when he hears Manaka's true feelings screaming throughout the sea. Screaming "I love Hii-kun!"
Tsumugu is the boy Manaka had her 'puppy' crush on. Because of this, Hikari disliked him very much. He didn't want the boy around Manaka or any other of his friends because he tends to get jealous when they are near him. He even got angry when Manaka called Tsumugu by his first name, saying it was perverted that she does it. In part 1, Hikari is very much against Tsumugu, saying that he won't allow the boy to take Manaka from him and during the school-race, he crashes into Tsumugu. Hikari gets upset when he realizes Manaka had shown Tsumugu their little secret, the fish head on her knee. It is then he realizes Manaka really cares for him and starts to try and be a little friendly with the other boy for Manaka's sake. While the anime progresses, Hikari began to consider Tsumugu as one of his close friends, even starting a water fight with him after Tsumugu build a water-whole for them. And at the end of part 1 (episodes 1-13) Hikari gained some kind of trust for Tsumugu as he leaves Manaka and his sister on his boat during the ceremony.
In part 2, Tsumugu is the only one Hikari generally feels would understand him. After Tsumugu asks why he hadn't gone to see his other friend, Hikari finally admits how he is feeling after waking up, to which Tsumugu gives the boy some advice. It would seem Hikari and Tsumugu are still good friends.
Kaname is Hikari's bestfriend. He grown up with Hikari and the others. In part 1, Kaname was the only person Hikari would listen to and would admit if he was wrong to. Kaname is the only one who understood Hikari as he was the one Hikari would go and ask for advice on when it came to his feelings or anything else. Kaname admires Hikari for his bubbly personality and sometimes wishes he'd rethink about his rash ideas. During part 1, Kaname says that he could never forgive anybody for upsetting Hikari and does his best to help him for he was the one who made Hikari confront his feelings and confess to Manaka. Nearing the end of part 1 (episodes 1-13) Kaname does his best to help Hikari with anything he can.
In part 2, because Kaname hadn't woken up get, Hikari found it difficult to confront his feelings since Hikari always had Kaname to help him but when Kaname does awake up, the two shares a deep discussion about Hikari's feelings. And once again, he helps Hikari figure out what he must do. Kaname is like a walking-stick for Hikari, Hikari relies on Kaname for a lot of things. When they travel back home under the water, Hikari insists Kaname comes along and when Hikari gets upset again, Kaname is the one to clam him down. Hikari begs desperately that Kaname helps him save Manaka when they finally find her in episode 18.
Chisaki is one of Hikari's close childhood friends. She had grown up with him and the others. Chisaki tends to mother the group greatly, and worries a lot about them. In part 1, it is revealed that she has developed a crush on Hikari and is a little upset at Manaka for being Hikari's main attention. She feels guilty for wishing Manaka to disappear after Kaname jokes about it making the girl wonder later on if she could really be next to Hikari. Chisaki gets shy around Hikari and blush easily, when the two are paired up during P.E, she is nervous about touching him and is hurt when Hikari notices and comments about it, saying that he wouldn't want to touch her either. Later, when the group has gone to town, she is taken from the lift with Hikari. She touches her arm where he had touched shyly and a little later realizes how much Hikari loves Manaka when the two speak about it in the lift. This makes her to 'hate' Tsumugu for changing everything. Right before the end of part 1 (episodes 1-13) Chisaki panics and confesses to Hikari while standing outside their old school. She admits that she can never beat Manaka, that Manaka is the one Hikari wants and understands that. She simply wishes that her and Hikari can stay friends to which Hikari response with a yes.
In part 2, after Hikari wakes up, Chisaki refuses to go see him. This upsets Hikari and makes him wonder why he should go see her. Later, the two run into each other and Hikari finds out that the only reason she wouldn't come to see him is because she didn't want him to be more upset that everything changed including her. Hikari reinsures her that the two will always be friends and says that she is the one who has never changed. This makes her happy and the two carries on being close friends.
In part 1, Hikari states that Miuna is a annoying little girl that likes to play dirty with her friend, Sayu. Through out part 1, Hikari helps Miuna and soon learns that she is the daughter of the man his sister is in love with and starts to form a 'brother' relationship with her. He even takes her downtown to help find a gift for his sister with her to which the others came along as well. It is shown that Hikari cares greatly for her as a younger sister as he teases her like a brother would do. Near the end of part 1 Miuna helps Hikari and friends with their project.
In part 2, it is revealed that Miuna has developed unrequited love for Hikari. She is now 14 years old, the same age and is happy about it until she realizes that Hikari is falling into depression and began to worry for him. Hikari holds no romantic feelings towards her, which she does not care for since she understand he wouldn't ever leave her and will always be the older 'brother' she needed. As she admits to Sayu, she won't change her feelings forcefully, they will change willing if needed, but for now she will support him and carry on loving him as she helped him to save Manaka.
Sayu is Miuna's friend and Hikari finds her amusing. He comments that she is a funny little brat and forgives her for destroying the Ofunahiki him and his friends worked so hard on. She is considered to be another one of his little annoying sisters. Sayu also has a crush on Kaname.
Akari is Hikari's older sister who he loves very much. At first, he is frustrated that she acts like their mother but finally admits it to her that she can't be his mother, he wants her to always be his sister instead of acting like a mother, To which she smiles and nods. Akari loves to tease him about Manaka, knowing that he is deeply in love with her and comments on it very much.
Akira is Hikari's adorable 4 year old nephew, also Miuna's younger brother. At first Akira is shy around his uncle and tries to avoid him but later, Hikari gets the young child to warm up to him and Akira comes to love Hikari as his uncle. The two plays all kind of games together around the table, often Hikari is chasing Akira.