Besides S02:E06 story refers to scene from S02:E01, where Ondina and Mimmi were listening to Sirena's talking with the Mermaid Council.

Sentences in the italics are Ondina's thoughts.

Of course I don't own direct dialogues from the series.

"I know what we were taught about mermen, but you have to believe me. Zac's no threat. He's harmless."

"Sirena, we've had this discussion. If he wasn't a threat, the pod would not have fled Mako."

"But he's our friend. Just ask Nixie and Lyla."

...

Sirena has always irritated Ondina.

They have never talked too much. Ondina hung out with Mimmi while Sirena spent her time with Nixie. Sometimes they had mutual lessons in the mermaid school and that was all.

During these common classes, Ondina observed Sirena. That mermaid was unbelievable! Annoying, naive, childish and sweet, definitely too sweet.

Once, when they were younger, Ondina complained to Mimmi about Sirena's unbearable attitude and her other traits, which were pissing Ondina off. "Why are you friends with me then? I'm very similar to her in many ways. Some of them you just mentioned. I hope that I'm not that vexatious to you," Mimmi said sourly.

"No, of course you are not vexatious!" Ondina quickly shook her head. "You are a great person and like a sister to me. You're both completely different, don't even compare yourself to her."

Mimmi only smiled. She knew that Ondina is stubborn as no other and it makes no sense to try to convince her of anything.

Time passed. Ondina's aversion towards Sirena thawed and transformed into some kind of interest. Interest later changed into attraction.

In the beginning Ondina wasn't sure what to do with this. It was still the same pesky Sirena. But she was also kind, lovable and generous. It finally tipped the scales.

Ondina decided to take the first step. Then it happened. Sirena, her friend Nixie and another mermaid Lyla have been banished and the rest of the pod, including Ondina, have been forced to set off in search of a new place to live.

…

The situation has hardly improved since then, in all aspects. The pod couldn't go back to Mako Island, as before. Ondina hasn't had any opportunity to talk with Sirena for months. But when such an occasion arose… she started irritating Ondina again.

Ondina heard every word from Sirena's interlocution with the Mermaid Council. What is she thinking? That the merman won't be tempted to gain more power, sooner or later? That he isn't a threat and, what is the most ridiculous, that he's her FRIEND? He manipulated her somehow (and the other two gullible mermaids), that's for sure.

Ondina had to do something. She had to protect Mako and the pod. She had to save Sirena.

…

A lot happened over the next few days. Unfortunately, things were mainly getting worse. Ondina and Mimmi have failed to take away the merman's power from a land boy named Zac. Furthermore, because of them, now also a land girl named Evie has been turned into a merperson.

At least this guy Zac appeared to be less dangerous than Ondina had assumed- he didn't seem to realize how threatening his new abilities could be and rather wasn't a bad man.

At that moment she and Mimmi reached Rita's grotto. They were just entering the part of the cave with seatings covered with cushions when they saw Sirena and Evie.

"What are you doing?" Ondina asked suspiciously.

"Teaching Evie to use her powers," Sirena revealed without beating around the bush.

Ondina felt like something was stirring inside her. Sirena was so reckless! Who knows if Evie is really trustworthy? "What's the point? She won't have them for much longer."

"You've figured out a way to change me back?!" Evie beamed.

"I was discussing a principle, not a time frame."

After this short exchange Sirena spoke. "So while she's got her powers, she may as well know what to do with them."

"And what makes you such an expert?" Ondina replied meanly. She didn't understand Sirena and it frustrated her. Showing to any human how their powers work was putting themselves in danger. Is doing this for a human girl, who only temporarily was a mermaid, worth it?

"I have a Moon Ring just like you," Sirena said and went back to tutoring Evie about moving objects with water.

Ondina and Mimmi looked at each other. Ondina was thinking that Sirena is incorrigible and Mimmi must be thinking of the same. Whereas Mimmi was thinking that Ondina is clearly in love and must have a hard time coming to terms with her own feelings.

…

They stayed in the grotto and watched the continuation of the lesson that Sirena was giving Evie. Obviously, Ondina wouldn't be herself if she didn't say something unpleasant again. At some point she began to reproach Sirena that she had received her Moon Ring from Rita and not from the pod. Mimmi tried to stop her from being unnecessarily nasty (especially for the person she liked), but to no avail. However, Sirena herself dealt well with Ondina's obnoxious behavior.

"I think you'll find that Rita knows just as much as any teacher in the pod. Maybe more," Sirena said emphatically, looking Ondina straight in the eye.

She is so cute when she tries to be firm, Ondina thought. "I very much doubt that."

"Ondina!" Mimmi decided to intervene a second time. She saw what was going on. Ondina's arguing started to become more and more of a flirt. But did Sirena perceive it that way too? Ondina might alienate Sirena to herself with such words. And that could bury the chance for their relationship to develop any further. As a devoted friend, Mimmi didn't want to allow it.

"Are you saying I'm not good enough to teach?" Sirena asked. She and Ondina kept eye contact.

I think that you are good enough. "I don't know what your skills are." I'd love to learn about your different skills.

"I'd be more than happy to show you."

Me too. "Then we're going to need something a little bit more challenging than that canoe."

Sirena squinted. She was eyeing Ondina as if she was also playing some kind of love game… It didn't escape Mimmi's close attention. Could Ondina's interest be mutual?

A moment later, two bantering girls looked at the large model of the ship standing nearby at the same time.

…

The first round was over. So far, Ondina emerged victorious.

"It just took me by surprise, that's all. Shall we go again?" Sirena didn't intend to let go.

"Why not? I enjoy winning," Ondina answered with great self-confidence. There was not the slightest exaggeration in this statement. She has always loved to win, no matter whether plays, competitions or in relationships with others. That was her, who has always dominated.

She couldn't help herself.

…

Ultimately, Sirena defeated Ondina after two consecutive bouts. After this Evie learnt how to move the canoe on the surface of the water.

It was almost evening. Evie had gone home about half an hour ago. Rita, who had appeared meanwhile, was showing Ondina how to create lightning inside. For this, they both came down to the lowest part of the cave with the pool.

Sirena was getting ready to go to work in the cafe.

"They're still there. I don't want to interrupt them. I think I'll exit through the main door upstairs," she said gently. "It's great that Ondina found out that Rita is a good teacher. Ok, it's time for me. Goodbye Mimmi, see you later."

She was already heading towards stone steps, when the brunette spoke suddenly.

"Sirena, wait."

…

"What? Ondina was flirting?" Sirena exclaimed.

Mimmi smiled timidly. She wasn't used to coming up with something but she had to help Ondina. If the whole situation was straightforward, her best friend would invite her chosen one to date some time ago. However, it was complicated by the whole thing about Zac's and Evie's obtained tails, specifically the different opinions Ondina and Sirena had. Because of that, Ondina was constantly angry with Sirena and kept her true feelings out of the way.

"I know, that sounds silly. I wouldn't be talking to you if I haven't suspected that you like Ondina as much as she likes you. If I'm wrong, please don't tell her about our conversation. She would be mad at me," but Mimmi was almost sure she was right. Almost. For Ondina's sake, she decided to take the risk.

"Yes, of course I won't tell her anything…" Sirena looked a little dazed. "But... you're not wrong. I like Ondina too… this way. How do you know about it?"

"I suppose I'm a good observer."

"You're a really good observer," Sirena clearly accentuated the word "really". "Honestly, I was also flirting. Or rather, I was hoping so" she laughed. "I saw what Ondina was doing. But I thought it might be only my imagination. I still wished that it wasn't."

"And it wasn't."

For a short while both girls smiled at each other and even giggled. They were happy- Sirena loved someone reciprocally and Mimmi was glad that Ondina's emotional crisis would finally end. But they also felt a bit embarrassed about their talk.

The silence was broken by Sirena. She thought that it had taken a lot of guts from the quite shy Mimmi to say all of this. So at least that's how she could repay for now- by not prolonging this potentially stressful conversation.

"Thank you Mimmi. You are wonderful. Tomorrow I'll tell Ondina what I feel. Unless she drops by the cafe today, then I can do it after the concert."

"No problem," Mimmi winked. "I will persuade her to come when they are done with Rita."