FleshCousin will have the same avatar as one of the players in the server, except for the face, which is a slightly distorted version of the default (Smile) face.

Its undisguised form has been shown to be a gray R6 rig wearing what resembles a piece of paper on its head with a crudely drawn smile face.  

Unlike other NPCs, FleshCousins are said in lore to be an entire species of entities rather than just one member. They aren't fully sentient, with animalistic intelligence comparable to birds or fish.[2]

The FleshCousin we see in-game is reportedly always the same one.[3] According to Yeuc_c, it is confused and lost.[4]

FleshCousins speak entirely in a word salad vaguely relating to things spoken or done to them, with them rarely speaking in a coherent manner. FleshCousins also have a tendency to mimic the speech of other NPCs on the elevator, although their words will still remain incoherent, much to the irritation of others.

Once the Happy Birthday? floor comes up, this NPC will first be seen as a cake topper. But once the Happy Birthday music comes up, the NPC says "It's my birthday!" and will soon step out of the cake and enter the elevator. FleshCousin mimics the player closest to them when the elevator door opens.

On the Funny Maze floor, FleshCousin can be seen wandering undisguised around the floor under the name "Fleshy". 

i Imagine the flesh cousin sounding like a distorted-glitchy and wobbly voice, theres a sound in the wiki of it making noise, so try to replicate that, anyways, HAVE FUN!

https://regretevator.fandom.com/wiki/FleshCousin#Current - Wiki!