I'm planning on running a Dark Heresy game & a player of mine had an interesting character idea. They want to play the reincarnated soul a rat which was the familiar of one of the shaman who reincarnated into the Emperor of Mankind, I really like the idea but I'm not sure how to represent it in the game, can rat souls become human souls? Would they maybe need some rat DNA in their genes? How would a rat-genespliced human be represented?

Uhhhhhhhhhhhh, I dunno. Whatever you want really. The way souls work, or the afterlife works in 40k is something that like everything else is shown to be vastly different from writer to writer. The one sorta constant is that living things have souls, and that these souls are in someway tied to warp energy and the warp. After that it's more up in the air depending on the writer or the fan. From human souls turning to formless warp juice after death, to them being feasted upon by daemons, to them being sucked into the Astronomicon. From only Eldar souls remaining intact post death, to human ones seeming to partially resurrect seemingly through force of will from time to time.

Your idea is a bit off the wall by most depictions. But animals do cast a shadow however normally faint in the warp. Maybe it being a familiar to a powerful psyker altered it in a way for it to stay cohesive, maybe it's just a warp thingie that thinks it was said rat. Whatever story explanation you want. Though mechanically I don't think there's much that need be done. It's a person now.

Please name your character Hamato Yoshi!

Well since it's a rat soul reincarnated as a human, it would be a normal human.

How about making your char a psyker with Shape Flesh (biomancy) power?

Yep just a normal human with some bizarre past life memories.

Certainly no need for gene-splicing and such, a transference of a soul from one host to another doesn't require DNA to be present from the previous life form.

As Noob says your idea is way offbeat and whilst not supported by the fluff there is nothing in it that really contradicts your idea. I say go for it and just use whichever fluff best supports your idea, it sounds awesome!

Thank you all for your opinions. I'm going to talk with the player & fluff it as the rat was psychically fused with warp energies to have almost human intelligence. When it died it was more individual than a normal animal & so loyal that it found a way to return to serve its master. (which was basically their idea in the first place) yes they will be a psyker idk if they want to actually look like a rat though.

That seems like a fairly cool way to do it. I hope it all works out.