Yeah, I can see Robinroot and Beltigre being chosen to represent Generation 7 in Smash.
Entermaid is the least likely of the three due to its mermaid-like body, which means it would have trouble moving around, especially when it comes to walking and running.
However, I think I've come up with a solution to that problem.
What if Entermaid could essentially "ride the waves" for its walking and running animations? Need a reference? Here's Water Kirby:
Water Kirby has a unique dash where instead of running, a set of water emerges underneath him, and thus he surfs for his dash.
I can imagine if it were in Smash, Entermaid would surf when it moves on the ground. That is, similar to Water Kirby, a set of water emerges from the ground beneath it, giving it mobility on the floor. It's walking animation would look like a more casual surf. For it's running animation, it would lean forward so that it can surf faster. For its dash attack, it would simply slide into an opponent with its arms outstretched.
Now, how would it jump? Again, looking back at the Kirby ability, Water Kirby has a dash attack where the water beneath its feet surges upward, giving Kirby more elevation while the water beneath it forms a wave that can damage enemies. Entermaid's jump could be similar to that, where the water beneath it surges up like a wave.
As for the second jump? I think Entermaid could do an acrobatic jump for the second one. Popplio can perform acrobatic flips, and while we haven't seen Brionne and Entermaid do these things, perhaps we could assume they are capable of that too due to their continued performer-like design.
Just a few ideas of mine if Entermaid were to be a Smash fighter.