I'm interested to hear from others about this too, having also only experienced it from the child's perspective...
I wasn't his carer per se but my father's mental illness affected all of us to a huge degree. I was the "responsible" one and bore the brunt of the family's problems. My mom had her own significant issues with illness and disability and I was more her carer that my father's, and my lil brother had (has) ADHD and depression. Little ol me with undiagnosed bipolar (I hid it because I had to hold everyone together and was just deemed "oversensitive" or just a bit "nuts") had to juggle the weird family dynamics since I was little, stop the fights and the tantrums, clean up the broken crockery etc. And I also had to explain to my friends why I couldn't come over to play because my massively depressed, paranoid, BPD dad wouldn't let me leave the house.
So yeah, I identify - and I really wish he had have faced up to his own problems. I eventually convinced him to go on meds by being sui and having to go on them myself - it was only seeing that he almost could have lost me that made him realize he had to do something for himself.
It's tough, I think a lot of kids do end up having to manage their parents' mental illness and I guess if the parent lacks the sort of insight (whether it's part of the illness of whatever) then the child can really get swallowed up by he parent's problems. Oh and of course there is always the joy that comes from realizing that you have inherited the parent's mental issues, yay for that

It's the stuff T is made of!