WARNING: The following contains spoilers for Episode 7 of Tropical-Rouge! Precure, "Here It Comes! Sea Fairy Kururun!" now streaming on Crunchyroll.
Mascot characters are a staple of the Pretty Cure franchise, usually following a simple formula: they are fairies from another world sent to find and aid the legendary warriors Pretty Cure, recruiting them to battle against evil. This makes Kururun of Tropical-Rouge! Precure unique, as not only is it introduced after all the main characters, but it's also basically a Pokémon instead of a traditional Pretty Cure mascot.
Unlike its predecessors, Kururun isn't a warrior, a doctor or even a well-meaning but klutzy sidekick, instead expressly being the pet of the Grand Ocean's current Queen. In the episode, Kururun is sent by said Queen to deliver some sweets to Laura and the Cures, allowing it to make the choice to stay with the girls at the end of the episode -- not entirely unlike when Ash or Goh befriends a Pokémon who then decides to stay with them by episode's end.
Kururun has plenty of Pokémon-like traits, from its obviously animal-like seal design to its ability to fit inside a pocket by entering the Mermaid Aqua Pot. The most notable similarity is the fact that, like all Pokémon, throughout the episode, Kururun is shown to only be able to say its name or some variation of those syllables, like "kuru" or "run," something that none of the other characters can inherently understand. These choices make Kururun stand out from among the franchise's other mascots -- which makes sense as it serves an entirely different purpose.
The majority of Pretty Cure's mascot characters have served as walking, talking exposition in addition to being fun characters. Usually, it is the mascot character who explains to the Cures what the threat they're facing is, why it's dangerous and how they have been drafted to fight it with their newfound magical superpowers.
In Tropical-Rouge! this role is filled quite handily by Laura, the cast's resident mermaid and queen candidate, who has not only been tasked with finding the Cures, but ensuring their success. With Laura being a central part of the group and the girls spending the majority of on-screen time together as opposed to in their homes with their families, it doesn't make sense to have multiple characters who can answer the girls' likely similar questions.
To that end, Kururun fills a much more literal role as a mascot -- it looks cute and is a creature for the girls to rally around and care for (making it perfect to sell tons of plushies.) While it can communicate through gestures and facial expressions, it does so with about as much success as Wobbuffet, requiring a creative mind to interpret what it wants. Fortunately, Minori, thanks to her love of books, is pretty good at inferring what the seal wants and quickly demonstrates her ability to think creatively.
While Kururun isn't the first non-speaking mascot, with plenty of the baby mascots failing to speak until much later in their series, it still carves out a lot of firsts for the franchise. Kururun doesn't appear to have any huge significance to the plot or unexplained abilities that could power up the Cures, and it seems unlikely to even follow in Pokémon's footsteps by being battle-ready itself. Rather, much like many other mascots, it'll likely be the cheering-and-supervising section, drawing the girls' attention to key weaknesses or giving them a reason to continue fighting after they've essentially been beaten. Either way, fitting with the show's themes of motivation, Kururun encourages the girls by its mere presence, making it a welcome addition to the cast.