Sven

"Who's my cute little reindeer?!"

- Olaf to Sven Sven the Reindeer (simply known as Sven) is one of the primary characters in the 2013 film Frozen.

Official Disney Bio

 * A reindeer with the heart of a Labrador, Sven is Kristoff’s loyal friend, sleigh-puller and conscience. He makes sure his mountain-man companion is the stand-up guy Sven knows and loves, and does so without saying a word. A few emphatic snorts usually get his point across. Life would be perfect if only Kristoff would lose that ridiculous reindeer voice he likes to use when speaking for Sven (as if reindeers really talk that way).

Physical Appearance
Sven is large brown reindeer. He wears a red harness around his body. Around his neck, back and hooves, he has tufts of white fur in a dusty pattern. The top of his head is a copper-brown color, unlike the rest of his body. If one looks closely, it is shown that he has brown eyes.

Personality
Sven is goofy, loyal, rather childish, and has the heart of a Labrador, as mentioned above. He spends most of the film by Kristoff's side, acting as his closest friend and conscience. And while Sven may not seem incredibly intelligent, he does have a strong moral character. Throughout the film, he constantly convinces Kristoff to do the right thing, including assisting Anna in her journey to find Elsa, and later return to Arendelle to confess his love for the princess after their adventure comes to conclusion. Along with Kristoff, Sven also seems to be greatly close to Olaf, enjoying the snowman's antics and eventually creating a close bond throughout the course of the film. As a running gag, however, Sven tries to grand a bite of Olaf's carrot nose. In the end of the film, he manages to do just that, but immediately returns it, showing he was merely being playful with his newfound friend, much to Olaf's delight.

Frozen teaser
Along with Olaf, Sven appears in the animated short teaser used as the film's first trailer. Here, Sven is feeding in a snowy forest when he hears pattering upon the frozen lake he's near. He turns and notices a carrot, and the mere sight is enough to get him smiling enthusiastically. The reindeer leaps onto the ice to retrieve the carrot, but the slippery surface makes it beyond difficult to do. Along with Sven, a snowman (Olaf) is also after the carrot, and a comical competition to retrieve it ensues. The two finally reach the center of the lake, where the carrot lies, and whilst Sven has a grip on it with his lips, Olaf grabs it as well, and accidentally flings it back onto the snow. Sven doesn't hesitate to rush over, leaving Olaf behind and saddened at the lose of his nose. Fortunately, Sven returns, revealing he was only trying to help Olaf, and the two set aside their suppose differences, becoming friends, much to Sven's absolute delight.

Frozen
As infants, Kristoff and Sven spent their time with the ice harvesters, learning the ways of the profession and tagging along whenever they can. One night, Kristoff and Sven notice the royal family (the king, queen, Princess Elsa, and Princess Anna), rushing over to the Valley of the Living Rock. Curious, the boy and reindeer follow behind to watch what all the commotion is about. As they spied, they're discovered by one of the trolls, Bulda, who decides to adopt the two orphans. From there, Kristoff and Sven became family with the trolls, officially taking residence in their valley. Years later, Princess Elsa is set to become queen after the death of the original monarchs, and whilst the kingdom is abuzz with royals from all around, Kristoff and Sven are out and about, too, but don't attend the castle festivities, likely because it's meant for royalty only.

Later that night, Queen Elsa was revealed to have great power over ice and snow, and as she flees her kingdom due to being called a monster, she accidentally curses the kingdom, causing an eternal winter. Meanwhile, Kristoff was busy buying equipment for his ice selling business, as well as carrots for Sven, from Wandering Oaken's Trading Post and Sauna, but gets kicked our for calling the store owner a crook, much to Sven's disappointment. They decide to sleep in a stable for the night, and as they sing their lullaby "duet", Princess Anna pays them a visit and asks Kristoff to guide her to the northern mountains to find her sister. Kristoff denies until Anna shows she's purchased the equipment and carrots he failed to buy. Sven was more than happy to help at that point, and the trio headed off. After facing a wolf attack, however, Kristoff wants to call it quits, but Sven doesn't want to leave Anna out alone, knowing she'll die, and persuades Kristoff to continue fourth.

Later on, they meet an enchanted snowman named Olaf, brought to life by Elsa, who offers to guide them to her ice castle. Sven immediately takes a liking to the goofy snowman, and the two form a bond. But Sven also takes a liking to Olaf's carrot nose, which he tries to eat a few times, but to no success. Soon enough, the friends find Elsa's palace, but he's unable to make it up the ice stairs. Kristoff tells Sven to wait outside, and when the others return, they're being chased by a giant snowman monster. Kristoff, Olaf, and Anna head off, and Sven goes in a different direction, meeting up with the gang at the bottom of a mountain cliff. At that time, Kristoff realizes Anna's been cursed when Elsa accidentally blasted her with magic back at the castle, and he leads the way to the Valley of the Living Rock.

Pabbie, the king of the trolls, tells the group that unless Anna performs an act of true love, she'll freeze to death. Riding on Sven, the group rushes back to Arendelle for Anna to kiss Hans, her fiancé, believing true love's kiss would break the curse. Once Anna is taken into royal care, Kristoff decides to leave her be, despite having deep feelings for her. Sven notices this isn't right, and tries to get Kristoff to turn back to proclaim his love. As he does, a storm was seen hovering over the castle, and Kristoff fears Anna's in danger. He and Sven rush off to the rescue, but the storm causes the two to separate. Sven survives, nonetheless, and tells Kristoff to keep going, much to Kristoff's delight.

When he reunites with the others, Sven finds Anna frozen solid, having sacrificed herself to save Elsa from the treacherous Hans. Fortunately, that's an act of true love, and Anna is reborn. The rest of the kingdom is thawed, as well, and Elsa reclaims her throne. As a reward, Sven is appointed royalty by Elsa, and welcomed into the castle as such along with Kristoff, and Olaf, who is given his own snow cloud to survive in summer. As the snowman enjoys the season, he sneezes, his carrot nose flying right into Sven's mouth. Sven playfully sucks it in, much to Olaf's sadness, but immediately places it back onto the snowman's face, proclaiming their friendship.

Sven is last seen with Olaf, Kristoff, Anna, and Elsa skating in the new ice skating rink of the castle's courtyard, enjoying the peace and freedom Arendelle greatly longed for.

Disney Parks
Along with Olaf, Kristoff, Anna, and Elsa, Sven appears in the winter themed version of the nighttime spectacular, World of Color in Disney California Adventure, making a small cameo pulling a sleigh carrying Olaf, Anna, and Kristoff.

Trivia

 * Unlike Olaf, Sven does not speak. However, as mentioned above, Kristoff comically does voice overs for the reindeer.
 * Whislt the filmmakers have noted Sven's behavior being based off that of a dog (or a Labrador to be more precise), real reindeer actually do pant much like canines. Real reindeer also scratch the back of their ears with their legs, much like dogs, and that was also incorporated into Sven's character.
 * Sven was originally set to be a one-antlered reindeer.
 * Sven is also similar to Pegasus from Hercules.They are animal sidekicks to the main protagonist both are shown in their younger forms and both of them act like a dog even though they're not.