His only appearances in Questionable Content have been in flashbacks.
Most of what we know about Mr. Whitaker comes from Faye in "The Talk".
The only thing known about him, outside of his relationship with Faye and her mother, is that he apparently liked to have bourbon in his milkshakes. This has led to some speculation that he perhaps was a "closeted drunk". However, Jeph Jacques has made it clear in referencing The Talk storyline that sometimes you never find out just why a person commits suicide:
Statistically speaking, most suicidal people do present signs of a depressed state of mind before killing themselves (and a significant number of suicides do not leave a note of any sort). However, everyone is different, and Faye's father (assuming her memory is infallible, which it isn't) was evidently either not manifesting "symptoms" or was extremely good at hiding them, for reasons that presumably went into the grave with him.
Imagine for a moment that the one person in your life you trust utterly and love unconditionally suddenly decides they no longer want to be alive, and follow through on that desire. What would such an awful event do to your psyche? How would it affect your interaction with other people? What if, because that person didn't leave a note explaining WHY they chose to end their life, you will never be able to definitively say that it wasn't your fault somehow? How would you get closure? How would you heal? Would you be ABLE to heal?
This is what has happened to Faye, ladies and gentlemen. As she says in panel three, it ruined her life, and the lives of her immediate family in turn. How do you pick up the pieces?
I'm not sure if I'd be able to. We'll just have to hope Faye can do it.
Suicide does more than end one life. It irrevocably changes (and often ruins) the lives of everyone connected to the person committing the act. So please, if you're feeling suicidal yourself or suspect a friend or loved one is, get help. Suicidology.org is a great resource for learning about the different warning signs of depression and other mental problems and has lots of information and contact numbers where people in crisis can find someone capable to talk to.