Mers usually mate for life.

Izuku was Shoto's mate, but Shoto died (Either from Endevwhore or Izuku's own original pod)

Izuku is sad and heartbroken for a few years ['Pets' are a thing in this AU - Similar to how they work in the 'Smol' AU, but not exact]

Izuku becomes Katsuki's pet because he stops eating and almost dies due to his grief

Kami 'owns' Shinsou and when he finds out about Izuku offers to have them meet

Izuku is intimidated by shin at first - Meaning he tried to fight Shinsou (Shin is a deep sea mer - Izuku is a beta fish mer)

Shin is immediately infatuated by the spunky little beta he was introduced to and begins trying to courting Izuku

Izuku rejects at first, and by reject, he keeps trying to attack Shinsou. Shin doesn't seem to get the hint, either that or he's ignoring it because he likes Izu. Kami and Bakugou keep separating them, Izu is fine with it, but he's still not eating like he should, and Shin keeps basically throwing tantrums at Denki every time he's taken away from Izuku (Even if he just got the shit beat out of him) So Denki is very conflicted.

Eventually, they set up this tank system in the common room where they can see each other, but can't get to each other so Shinsou stops getting the shit beat out of him.

Once they become more accustomed to each other via seeing each other through glass, they get reintroduced. Shinsou is back on his courting bullshit, Izuku is less aggressive and doesn't outright accept Hitoshi's advances, but he doesn't shove him away either. He just kinda lets it happen.

Like I said Mers usually mate for life, but on very very rare occasions will a mere take a second mate. The phenomenon is so uncommon that it's only been documented about 7 times total, 3 in the wild and 4 in captivity - none of them consisted of a beta mer because of their aggression finding even one mate can be extremely difficult for them, so finding a second one is very unlikely.

That number eventually goes up to 8 and the number containing a beta mer goes up to 1 because of Izuku accepting Hitoshi's courting gifts.

Mers that aren't of the same species mating are also very uncommon, they usually stick with their same species, but not always.

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Feel free to leave suggestions and scenarios you'd like to see