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The siren is an amphibious humanoid. They are mostly like humans apart from the limbs: Their arms are large and flipper-like, being used to swim underwater. Their leg is short, and they always have type 7 sirenomelia (where there is only one leg with a single femur). They evolved from a hopping humanoid with fused legs, which evolved from a typical walking humanoid
How would they move on land?
The closest real mammalian counterpart from a creature like yours in terms of overall bodyplan (arm flippers, feet acting like fins) are seals and sea lions, and sea lions don't quite fit the bill since their 2 legs are still very much capable of acting like decently standard legs, unlike the legs of a seal which are essentially locked in a perpetual backwards position serving more like actual fins with basically no ability to move properly on land.
Therefore, your creature probably moves much like a seal, aka bouncing their way around.
A proper layer of fat might make this easier as well as allow for better insulation against colder temperatures.
Flipper crawl. Not much extra needed for that sort of thing if their 'arms are large and flipper-like'
