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 probably 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 Hikari's mind. He wishes to find her quickly. 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 wanting Hikari to kiss 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, Manaka began to notice odd things. She noticed her memory has blank spots and this troubles her greatly. She grows scared as she realizes she 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. Manaka notices whenever somebody says something loving to her, in return she feels noting, 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!"
In summary Manaka and Hikari love each other.
Kaname is a childhood friend of Manaka who had grown up with them. Kaname is like Manaka's older brother. He helps the others to protect her and is also one of the few Manaka would go to for advise. He speaks with her many times about her feelings. Kaname is one of the people Manaka admits that she is scared to go to sleep to.
Tsumugu is a human boy Manaka had admired. Upon first meeting him, she noticed he had a great connection with the sea and grow to admire and care for him. Stuck in-between being in love with Hikari and Chisaki also like Hikari, Manaka thought it would be best if she had fallen in love with somebody else and mixed up her feelings. Manaka mixed her admire for the boy to be feeling of love as she tries to explain to Chisaki at one point, hoping the other girl would help her understand her own feelings better. At first, Manaka was certain she had fallen for the boy as he had saved her and also showed a little interest in her, but as the series progressed she began to notice he was like the sun. Manaka had always admired the sun, thought the sun was beautiful and warm but she did not love the sun, she had loved the sea more, thus resulting in Manaka sadly accepted that she did not love Tsumugu as she wishes she did. Manaka desperately wanted to be in love with him as she thought it would be better for Chisaki as the other female liked Hikari so much. But upon realizing she did not, in fact, love Tsumugu as a lover but indeed love Hikari, Manaka tried to keep their friendship safe. Tsumugu found Manaka when she was crying after Hikari had confessed to her. Manaka desperately told Tsumugu everything about her admiring him and loving Hikari, that Tsumugu was the sun and Hikari was her sea. Tsumugu finally asked if that meant Manaka was in love with Hikari to which Manaka nodded and said that she had only tried to find interest in somebody else because she had learned of Chisaki's feelings. Tsumugu told her not to give up to which Manaka begged him not to tell anybody, this meeting becoming a small promise between the two.
Manaka knew something had happened that night, she tried to remember what her conversation was with the boy but every time she tried, she would hear the sound of crashing waves, as well as her head, hurting and her body feeling empty. Tsumugu rushed to her home later and showed her this fish head in hopes to help her recall what had happened between them, disappointingly it did not help. Tsumugu then finally told Hikari the truth about that night, that Manaka's feelings had never been to for him but for Hikari, himself. That Manaka had admitted she had simply admired Tsumugu while being in love with Hikari.
Tsumugu came to care for Manaka as a younger sister, though he still calls her Mukaido instead of Manaka. Manaka comes to Tsumugu whenever she needs advise and loves the boy like a brother.
Chisaki is a preciouses friend of Manaka's. Manaka loves and adores her very much, she looks to her whenever she has a problem, may it be big or small. Chisaki treats Manaka as a younger sibling and sometimes a bother, as Manaka is always in the way of Chisaki and Hikari becoming closer. At one point Chisaki goes as far as to wish for Manaka to disappear. When Manaka overhears that Chisaki has romantic feelings towards Hikari, she believes that it may be best if she was to step aside and allow Chisaki to go after Hikari. Watching Chisaki and Hikari grow closer eats away at Manka, but still, she persists in her efforts of trying to help Chisaki get closer to Hikari, regardless of the pain it causes her. This creates a rift between Manaka and Chisaki. Chisaki feels as if Manaka is trying to force her to be with Hikari at a time when Chisaki isn't ready to confront Hikari on her feelings for him, do to a looming fear of change. Manaka shows interest in Tsumugu and digs deeper into these feelings believing it is best for Chisaki. Unaware that Chisaki has noticed Hikari's feelings toward this and wishes Tsumgu would leave Manaka alone. Manaka does a lot to make Chisaki happy, at another point she even went as far to drag Chisaki onto the old school roof to watch the salt spring, something Chisaki had been upset about when they were children. Chisaki slowly notices that Manaka is leaping far ahead of them, wishing to explore everyone's surrounding.
Chisaki is panicked upon realizing that Manaka was used as the sacrifice for the sea god and thinks that she should take Manaka's place, so Manaka can live a normal life. Chisaki realizes that Manaka would never forgive her if she was to do so. Upon finding out Manaka can no longer love anybody, Chisaki wonders if that would be best for the girl as Chisaki herself as now fallen in love with Tsumugu and thought that Manaka had also been in love with Tsumugu. Chisaki holds back on all her feelings, trying to finally be a good friend for the young girl, but fails as she realizes she is only making it harder for Manaka.
Upon finding out that Manaka had never been in love with Tsumugu, only admired him and cared for him like a brother. Chisaki feels generally happy, especially when she finds out Hikari's love has never been one-sided and that Manaka had held back when she had learned of Chisaki's feelings. Because of this, Chisaki feels torn. Half because she can now love Tsumugu freely but also sad because it is her fault that Manaka and Hikari had been suffering.
Akira is Hikari's 5-year-old nephew and Akari's son. Akira is one of the few people who was first to see Manaka after she awoke. Manaka had always wanted to be a mother so she grow attached to little Akira straight away. Unable to love, Manaka notices no matter how she tries, she can not feel anything for him but a strong sense of care. Manaka spends most of her time with Akira and becomes one of the few people he'll listen to, she plays a lot of games with him and in her spare times they draw together. Akira cries when Manaka leaves the house for awhile, calling out her name which he can say probably. "Nanaka!" While Manaka is preparing herself for bed, Akari and Akira enters the bathroom and Akira shyly gives her a beautiful picture he had drawn within pre-school, Manaka looks upon it and her chest straight away begins to hurt, her head fills with the sound of rolling waves as she realizes Akira had developed a small crush on her. Unable to handle the pain she felt, Manaka retreats to her bedroom, hugging the picture tightly. The next day, Akira sees Manaka's necklace and finds out that it is filled with Manaka's feelings, feeling hurt and lonely, wanting Manaka to show him love, he steals it only to trip and have it fall into the water, the other straight away dives in after it. When everyone is preparing for the festival, Manaka smiles at him and says that she loves him too, also thanking him for the beautiful picture. This makes him blush from embarrassment and shyly runs away, not before giving 'Kancho' to her of cause. His way of saying 'I love you' When he sees Manaka fall into the sea, Akira cries out her name and is happy when she returns.