Writing Difficult Characters

I've been working on a writing project off and on for about 6 months. I love it because the whole purpose of this piece of writing is to be as ridiculous as possible while still maintaining a margin of believability. Since it's a romance, this is much easier to do.

The reason that this has been an off and on project instead of just on is for one reason alone. My difficult character. Her name is Tina and she has the amazing ability to stop conversations dead with her inanity. This wouldn't normally be so difficult, but I'm in the beginning of my first rough draft. My writing style is such that my character's personalities and quirks are rather nebulous until I get to the end of the first draft (or sometimes even later). Because I don't have an entirely clear picture of my main characters, it's really hard to guess how they might react to Tina's ridiculous statements, thus stopping my writing dead. It takes multiple days, sometimes weeks, to come up with an answer to the question "How would my character react to this?". And since Tina is actually a fairly important character in the beginning, I can't just kill her off (though I do this often enough in my imagination).

Anyone else have this issue?? How do you deal with it?