One of the witnesses the jury will not like. They may even despise him. I wouldn't blame them at all. Once he's done testifying, I'll tell you why they won't and how he turned out.
I'll tell the jurors about it during jury selection. I'll explain why they won't like him. I'll talk about how we are not asking them to decide if he should come to their house for Thanksgiving dinner, but whether they can decide if he's telling the truth or not despite their feelings toward him. It's best to lay your major weaknesses in front of the jurors early and gauge their reaction. This way we can make informed decisions on which jurors to select.
Some jurors will be able to, some won't. Those that won't will be excused (hopefully!). We'll see how it turns out next week. Luckily, he's only one of twenty.