So my group and I were brainstorming a new RT game when I thought up using Pelagers. The Abhuman fishmen of the Imperium! This was mostly cause I had gained a recent fascination with the Carcharodons Astra space marine chapter, but we lack Deathwatch's Core Rulebook and they wanted to play RT for all the batshit setting breaking nonsense you can get up to in the Expanse. I decided meh why not but managed to wrangle a concession of the GM so I could play an angry sharkman who bisects people for the Emperor! Unfortunately this bit from the wiki and Lexicanum is all we have on them.

• Pelagers ( Homo sapiens oceanus ) - Pelagers have either evolved or were genetically altered during the Dark Age of Technology to live on Ocean Worlds , those habitable planets of the galaxy almost entirely covered in liquid water. They possess a set of both gills and lungs, and other adaptations for an aquatic lifestyle such as webbed hands and feet. They are a common sight on such worlds of the Imperium, but virtually unknown everywhere else.

For Clarification we're talking something like Kisame from Naruto or Costner's Character from waterworld appearance wise. Still recognizable as human not straight up Deep Ones or Mer people(though fighting Deep Ones or Sarhaduin would be fun).

Does anyone know where I can get rules for making a character with this origin?

When I wrote my own ruleset a while back I created a bunch of abhumans including gillmen:

Gillmen - A common form of mutation on water worlds is the addition of Gills in the neck of a human giving them the ability to breathe in both air and water - more subtle alterations give them the ability to survive the pressure 3x that of normal humans and they do not suffer decompression sickness.

No base stat alterations.

Imperial Acceptability 8 - abnormal but acceptable (on Water worlds they are fully accepted and count as 10 ).

Potential Tech Level- Roll D10 - 1-2 - Low 3-7 Average 8-10 High.

Initial Fate - 3

Height 5' 6"+ 2d6 inches Weight 150 + 4d10 lb

If you are staying true to the Rogue Trader ruleset you'll need to create a new starting world type for Water world - I would suggest increased toughness for their denser flesh and resistance to pressure but also some penalties while on land (clumsiness - Ag modifier ) but give them the natural ability to function in zero G without penalty due to their experience moving in 3 dimensions in the water (and obviously give them swimming skill).

Abhumans are fun so good luck!