I think it's probably fairly safe to assume that both Yuna and Taiga have claimed true. Both Jonathan and Angel are pretty reasonable character choices for this game, and I think it would be incredibly risky to falsely claim one of them on the off chance that someone else was that character for real. It's always possible that one or the other of them is lying, but right now I'm going to operate from the assumption that they are not.

So let's evaluate, starting with Angel.

http://en.wikipedia.org/wiki/Angel_(Buffy_the_Vampire_Slayer)

Within the context of this game, Angel is an interesting choice because when he's good he's very very good, but when he's bad he's horrid. By which I mean straight up murdering people for kicks and giggles. You can read all about him on Wikipedia if you'd like, but the gist of it is that Angel was a super evil vampire who got "cursed" with a soul, which turned him good. So then he was super good for a while, but then he broke the curse by experiencing a moment of true happiness by doin' it with Buffy and he turned super, super evil again. And then she killed him, and then he came back and had a soul again, and then he got his own tv show where he ran a private investigation firm or something. So the First Evil's quote about shifting people backward or forward in time to make them more "fun" fits PERFECTLY with Angel's on/off switch mortality thing. He's honestly the first character I thought of.

Now Jonathan is an interesting character because while he is also morally ambiguous he's not nearly as evil as Angel. But he's never anywhere as good as Angel either. Jonathan is a regular human kid who gets mixed up in some bad stuff. He spends the first couple of seasons being the wimpy kid butt of the jokes type thing, but he learns some magic and starts causing trouble. In Season 6 he joins up with these two other nerdly types to take over Sunnydale. Full disclaimer: I have only watched halfway through Season 6, so while I didn't actually see how it ends, it's on Wikipedia. Anyway, the three of them end up killing a girl on accident, and Jonathan is the only one who feels bad and more or less turns good, even though he was a pretty shitty bad guy in the first place. And then he dies in Season 7, and that's pretty much it. So while I have a harder time imagining Jonathan as a mafiascum type evil person, he did spend a fair bit of screen time as an antagonist, and plotted to kill Buffy at least a little bit.

http://en.wikipedia.org/wiki/Jonathan_Levinson

So while Jonathan was definitely not ever a Scooby and Angel definitely was, Jonathan was never as mustache-twirlingly-evil as souless Angel. Both have the potential to be roled, both have the potential to be scum. That's all I've got.