"You stupid, naive FOOL! You were supposed to seduce him, not the other way around!!"-Ursula
"I won't let you take him! I've sacrificed so much to be with Eric and I will not hand him over to someone who doesn't even love him!" -Ariel
"You think I intended this to happen?..." -Serene
... Yup. You send a young guppy to do a woman's job, and look at that! She falls for her
In my verse with my OC Serene, Ursula first sends her apprentice to seduce Eric away from Ariel. In the beginning it was all about sport, playing keep away with the little fish princess. That is, until Eric began to show genuine interest in Serene, much to her shock. I mean yeah, that's what she was sent to do, but she didn't think the prince would actually start to like-like her .... Nor did she anticipate the growing mutual feelings. In the end though, Ursula comes as Vanessa and hypnotizes Eric and the wedding proceeds as in the movie. Only unlike Ariel, Serene was on the wedding boat as "Vanessa's" bridesmaid and carried her train. A bit of emotional punishment for falling for the prince. When Ariel's and Ursula's true identities are revealed, Serene's tentacles come back as well and she attempts to flee the ship, but is captured by the shocked and terrified wedding party.
In the end, Eric has to make a choice- the first girl he loved that saved his life, or the other girl he loves and shares so much more in common with. (While Ariel was mute, Serene was able to have actual conversations with Eric and they got to know each other pretty well and found they had more than a few common interests).