Sandy, Tori and Cat Friendship
To be completely honest, Sandy thought she was seeing things when she saw that the two human girls can walk around underwater without any swimming and diving gear, she just couldn't believe it and never seen anything like it before. Before the girls had a chance to tell her, she ran some tests nonstop for a few hours until they finally stopped her and told her how they were able to breathe underwater. She processes it as they explain it to her and completely understood.
Sandy, Tori and Cat all get along with each other and them being all land creatures and though not the same species, its easy for them to understand each other. Sandy also becomes good friends with them but seems to be closer to Tori due to their maturities. She still adores Cat and thinks she's the cutest thing but doesn't like when she touches most of her science projects and inventions, Sandy scolds her every time Cat almost breaks or tampers with them.
The trio also share a musical talent as Sandy can sing and play the guitar and saxophone. Sometimes they even sing songs together.
She also tries to help them on some problems that they have, if it's school related and Cat wants to pass a math or science exam and needs help studying, Sandy, if not busy, won't hesitate to tutor her since Sandy is very good at those two subjects.
Or if they're being bullied and need to defend themselves, she'll most likely offer karate lessons to them if they feel like they need it depending on if the bullying is physical.
Sandy is very caring towards the girls but also will give them tough love and honesty when necessary.