When someone refers to someone as a dolphin, it is implying they are appear to be genuine to your face, while by nature they are malicious.
Girl 1 - “did you talk to Nicole last night?”
Girl 2 - “yea I did, she was really nice to me”
Girl 1 - “oh really I overheard her talking to jake saying how annoying you were”
Girl 2 - “eh she’s such a dolphin”
In the gacha gaming community, a dolphin is a person who spends a moderate amount of money for their gacha games on a regular basis, but nowhere near the levels a whale may be able to attain.
While mileage may vary depending on community and game, dolphins are estimated to spend a few tens in a game per month on average.
"This game has messed up my finances. I'm done being a whale for pictures of waifus, now I'm downgrading to a dolphin for my bank account's sake!"