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Rotates merfolk powers...
I feel like there's slight differences between different types of merfolk ofc but here's what Archie (and most merfolk) can do:
Manipulate water; move it around, levitate it, increase or decrease it's temperature, turn it to ice, evaporate it etc
Being immune to having his mind read by Psychics, be it Pokémon or people
Telekinesis.
Becoming invisible for a limited amount of time.
Shapeshift between forms (human and merfolk)
Enhanced voice / 'singing' capabilities. Think sonic screeches and siren song. Siren song gives them the ability to manipulate people's thoughts and actions through words (or singing if you're extra like that I guess). Most merfolk agree to never use that ability, and it's highly frowned upon in most circles. Merfolk who regularly travel to the depths have build up a tolerance / resistance for it though. (I HC the depths of the ocean are extremely hazardous / unfriendly and therefore super dangerous. Doesn't stop some brave souls from wandering there though.)
Enhanced senses underwater.
Can understand Water-types
They're one of the slower healers among supernatural beings. It makes venturing into dangerous situations extra risky.