Riven is the stubborn maverick of the group. He is Musa's love interest. His first appearance was in Season 1 Episode 1. He often argues with Musa and even end up her breaking up with him.
Season 1-Riven is introduced as one of the Specialists that Stella brings along with her to Earth. He is shown as standoffish and shows an immediate dislike towards Sky. They are both competitive in their training and neither wants the competition. Riven originally begins to like Musa though flirts with Bloom a little, just to make her, and possibly Sky (then acting as if he were Brandon), mad.
During Friends in Need (RAI version cited) when the Minotaur summoned by the Trix knocks him through a wall, Darcy spies Riven and comments to the other Trix that she senses a dark power coming from Riven. Icy states he may be useful later on. Riven also catches Musa after the Minotaur sends her flying through the air (this was cut in the 4Kids dub) he looks rather angry about it and states "Now this is my kind of party" At this point though you can't tell if their feelings are mutual or not. At the end of the night he tells Musa and Flora that the next time they have a party to leave him out of it.
In 4Kids, after having a spell put on him by Darcy during the Race for the Rose with help from Icy and Stormy he 'falls in love' with Darcy. He spends most of the first season as their spy until he is betrayed by them. He escapes from his cell in Cloud Tower and helps the Winx and rest of the Specialists win the battle.
In RAI, Riven chose to side with the Trix of his own free will. He was never mind controlled, which makes his actions even more despicable as he willingly betrayed his real friends on his own. It was only until he was told by the Trix themselves of their main goal of taking over Magix did he finally realize what he had been doing and gets jailed after he outlived his usefulness. While held prisoner, Darcy mentally tortures him with shadow clones of Riven appearing in his cell to mock him, then they all turn into a hideous monster, where he was forced to take a long look at himself to answer for what he had done, he later is able to escape and break the cell of ms.Griffin, the monster go after him and he jumps from the castle.
It is revealed that Riven is extremely smart and he knows his studies well at Red Fountain.
Upon Riven's return to Alfea with Brandon and Stella he's seen standing outside Musa's dorm window. In the 4Kids version he tells Musa that 'They don't teach this kind of thing in his school, but he wants to tell her he likes her'.The Cinelumè dub and the RAI Italian both versions have him tell Musa he was looking for Timmy, using it as an excuse to see her because he chickened out on his real plan to tell her his feelings.
Season 2-It appears that over the summer holiday Riven and Musa have come to some kind of understanding with each other. They seem close enough at times and a relationship may be starting.
During the concert at Red Fountain Riven can even be seen smiling when he sees Musa on stage. However there are some setbacks. In "The Show Must Go On" it's revealed that Hoboe (Musa's father) isn't too fond of Riven. Hoboe refers to Riven as a nearer do well and wishes Musa had found herself 'A Prince like Sky'. However, Stormy decides to take out her revenge on Musa for ruining her spell to steal the Codex from Alfea and breaks in to the stadium to attack Musa's dad.
When Musa- realizing she can't use her usual attacks on Stormy because of the effect they'll have on everyone else- tries to encourage the audience to sing with her it's Riven who inspires the crowd by yelling, "Come on you idiots- do what Musa says! Sing!" And he helps Hoboe up off the ground after Stormy is defeated. Hoboe realizes that Riven isn't as bad as he seems and apologizes.
In "Face to Face With The Enemy"Riven gives Musa a hug and kiss telling her that she needs to come back to him.
In "The Phoenix Revealed" Riven leaps in front of Musa to save her from Lord Darkar's attack. When everyone returns from Shadowhaunt Aisha spins Musa into Riven on the dance floor and after blushing he allows Musa to lead him on to the dance floor.
Season 3-Riven has become a kinder person since he started to date Musa. However, he definitely has not lost his jealous, hot-headed side. Nor is he able to confess his true feelings to Musa which leads to problems. Helia tells Riven that if he does not stop acting like a jerk Musa's going to leave him.
When the Winx travel to the Crystal Kingdom (Citation needed) he spies Musa standing in front of Nabu while watching the video that Flora sent to Helia. Assuming that Musa is kissing Nabu, while in reality she was placing handcuffs on his wrists, Riven gets understandably upset and manages to get to where the Winx, Timmy, Sky and Brandon are
After using Glim to help him find where the others are, Riven confronts Nabu, accusing him of kissing Musa, whom he calls his girlfriend. They get into a fight- which Riven loses. But Musa is encouraged by the fact that Riven got upset enough to come after her and fight for their relationship.
Secret Of The Lost Kingdom-During an attack by Mandragora, Riven was put under an enchantment when Mandragora's bug bit him and injected a kind of venom in him which caused him to become Mandragora's puppet. Later on in the movie, when the Winx are all blocked within Obsidian. Riven starts to fight against the Specialists due to Mandragora's spell. He ends up hurting Musa and when he sees her unconscious on the ground he remembers all the good times he spent with Musa and how much he loves her, thus breaking Mandragora's spell with his love for Musa. The venom the bug released in Riven's blood when it bit him was removed when black tears, due to the presence of the poison Mandragora's bug in them, fell from his eyes he swore that shes going to pay for this and atacked her,that end up killing her.
Season 4-The Specialists, along with Riven, are inquired to spy on the girls while they're on their mission on Earth, in case they get into danger by request of Miss.Faragonda. Riven and the others then get jobs at the Frutti Music Bar.
In this season Riven and Musa's relationship hits various critical points throughout their missions on earth.
It all begins when Andy suggests Musa pursue her music talents as a singer at the Frutti Music Bar, and Riven disapproves. When Musa gets a chance to work with a music producer whom Musa is fond of, Riven becomes not supportive, enraging Musa, thus causing her to break up with him. Nabu then helps Riven win Musa's heart back, which leads to rather humorous results.
Despite all this drama, Riven also helps in many fights against the wizards, showing that his skills improved tremendously over the course of the series, and he started to use his weapons and skills more tactically, making his fighting style not so sloppy.
In the end Riven begins to show support and hide his jealousy. At one point he even wanted to buy Musa some time to take part in a music contest, so he and the Specialists sold with a magic-enhanced number of their own. At the end they agreed that they would give their relationship another go. 
