Stephanie: Stephanie is Mia's best friend. They seem to get along better than the other girls, and always seem to make time for each other. Mia is sometimes genuinely concerned about Stephanie, especially if she is ruining her sleep schedule like in Game On or putting herself down over a loss of a competition seen in Heartmore and Go for the Gold. On the plus side, their relationship seems to be very mutual as Mia is seen helping Stephanie when she gets hurt as seen in The Shadow Group, Heartness Monster and Game On. It also seems that sometimes Mia can seem a bit protective of her, stalking her in Stakeout as she is playing basketball with Kacey, calling and texting her a bunch of times to see if she is okay in Game On.
Andrea: Mia and Andrea sometimes get into silly arguments. In "The Need for Speed" Mia becomes a bit harsh as Andrea lunges at her for "stealing" her spot to drive George in the semi-finals. It can be inferenced that they don't have the strongest relationship, as Mia is sometimes seen intimidating her obnoxiously in Explorers Day and Winner Sings All. She also tends to blurt out her most sarcastic responses when Andrea is around, such as when Andrea lost her voice for the day, Mia makes a comment about her driving.
Emma: Mia seems to tolerate Emma, no matter how far her head is in the clouds. She seems to help Emma get herself back into reality, and also help her figure out common sense things. In Zobo 2.0 Emma can be seen interrupting Mia as she tries to get a sentence out, which makes her tired of even trying. Mia also wants her to realize that Kacey is not someone they can trust in The Real Deal as Emma starts jumping ahead to tell Kacey about their plan.
Kacey: Mia first meets Kacey from Dr Donna in the vet clinic. She seems to be confused as to how Kacey knows her name, as Mia is squeezed into an uncomfortable embrace. After Kacey introduces herself to her friends, Mia is still a bit confused as to how and why this random girl appears out of no-where. Things don't really get better as Mia is pissed when she has to take a trip into the ocean with the girl, making glances and uncomfortable remarks at her. Over the process of the day Mia spies on all her friends that Kacey interacts with, noticing how she won't open up to them about who she is. At the end of the day, after being reprimanded and scolded that she should try to like Kacey, Mia invites her out to ride horses. They seem to get along well as Mia explains that it might be scary to make new friends. But things take a turn back in The Real Deal as Mia is still unsure about Kacey and is nervous about letting her in on secret plans. Once Kacey's identity is revealed-instead of making a rude remark, Mia accepts a handshake from her and they settle out as friends. After that point, Mia seems to really care about her new friend, always making sure she is comfortable in group settings and even protecting her will underwater from Carter in The Art of Being Carter Greene. Mia is sad when her new friend has to head back home that she gives her a warm hug.
Vicky: Mia doesn't really have anything against Vicky when they meet in The Daft and the Furious. She only views her as an opponent in the Grand Prix. Mia then agrees that the team should let Vicky drive George since her arm is broken. During the events of Explorers Day, the roles are reversed and Mia replaces Vicky as the lead mermaid in the play.
River: Mia and River are a cute couple. Mia first meets River in "Save the Bay" when he compliments Emma on her doc. She then approaches him at the party because she notices him staring at Ethan dancing with Emma. In "Follow your Passion" Mia blushes at the sight of River being called over to their table, and lunges at Stephanie for inferring that she was the school directory. During a picnic in the park in "Miarella" River asks Mia to the dance. At the end, after she runs away, he meets her by her horse stables. He proceeds to tell her how much he admires her by sitting down next to her to glance at the stars. In "Part of The Herd" Mia is shocked when her dad invites River to come to the jungle with them. On the plane during rest time, Mia leans her head on River's shoulder. As their summer jungle trip is coming to an end, River gives Mia a gift of an elephants foot and Mia holds the gift tightly to her chest. In S4 Mia and River go into a secret circus to find a horse, and freak out together over a creepy girl in the circus. Mia hugs him tightly to her chest after he announces that he is leaving HLC to pursue new adventures. She wipes a tear as the truck pulls away.
Daniel: Daniel is Mia's older brother. In Into the Woods, Daniel is getting ready to leave for college, when Mia breaks down and hugs him tightly. She then goes to her treehouse to comfort herself by looking at the pictures of her and her brother when they were little. In "The Grand Prix" he surprises her by being there for her when her team wins the race. In "Never Stop Exploring" Daniel gives Mia a treasure map by her house and she goes on a search to find the treasure. When Mia thinks she found the treasure, Olivia explains that the gold coins are fools gold and that it was a trick. In "Road Trip" Mia and her friends take a road trip up to Daniel's college to see him. And Mia is super happy.
Ann and Angus: Mia's parents.
Ava: Mia's cousin. (Only seen in Lego form)
Autumn: Daughter.