My theory is that Rhoun and Marruk are either exes or got into some sort of accident, and Marruk blames Rhoun for having his artificial eye. Marrow just has his heart because of a birth defect, not any mysterious backstory (I think). They all need their neural devices to speak to the protagonist, though. It's probably not a coincidence due to that. There are so many ways Marruk and Rhoun's backstories could turn out, so I'm waiting for the next few updates to learn more about them as characters.
Hold it!
Here's my theory: Based off of you saying them being exes, here's what I think.
I think that they were together, but then over time, due to Marruk's constant mood swings, and Rhoun's, uh... Tendency to space out a whole lot, along with him being somewhat scatterbrained, those two just sorta fell apart. Marruk felt like he was to blame, and neither knowing what to say to him, nor what to do about it, he tends to lash out the most on Rhoun, because he's the one who seemingly "hurt Marruk".
Do you have a favorite character? In furry VNs, I always go for the one who seems abrasive and cold (Marruk here). Usually, the anger and tough persona came to be because of intensive stress or having had to put up with a lot in life. Eventually, they open up to the MC and become friends or even more. The romance aspect is nice to see because there isn't as much representation in mainstream media that resonates with me on the same level as these visual novels. I'm going with Marruk for my first play through; seeing as he's the type I usually pick, he's hot, and he seems to understand Tucker.