The nameless man in the yellow suit is a bad guy—make no mistake about it.
Even before we know how slimy he is, the narrator makes it pretty clear that he's kind of a creeper:
His tall body moved continuously; a foot tapped, a shoulder twitched. And it moved in angles, rather jerkily. But at the same time he had a kind of grace, like a well-handled marionette. Indeed, he seemed almost to hang suspended there in the twilight. But Winnie, though she was half charmed, was suddenly reminded of the stiff black ribbons they had hung on the door of the cottage for her grandfather's funeral. (4.4)
Winnie's sense about him turned out to be true. This guy is up to no good. He shows up out of nowhere, does all kinds of sneaky things, is generally really creepy, and gets in the way of all of the characters we're rooting for. In Treegap, a small town where everybody knows your name, this guy is definitely out of place.
We do have to give him props, though. He set out to find out the truth about the Tucks, and he succeeded. Even if it left him dead in the end.
We can tell a lot about Yellow Suit Guy by his attitude toward the immortality spring. Remember what Tuck tells Winnie about becoming eternal? It's "'something you don't find out how you feel [about] until afterwards'" (12.11). For Yellow Suit Guy, it's definitely not afterwards yet.
No matter what their final views on immortality are, the Tucks have taken a lot of time to think about the ethical and philosophical questions that go along with it. Yellow Suit Guy? Not so much. In fact, we're not so sure he cares about immortality at all. He mostly just wants to make money off the whole deal:
"[I]t's very simple. The wood—and the spring—belong to me now." He patted his breast pocket. "I have a paper here, all signed and legal, to prove it. I'm going to sell the water, you see." (19.21)
Someone's not taking this whole thing very seriously, that's for sure. This guy really doesn't care about the Tucks' predicament and their way of life.
Which is more threatening: "the man in the yellow suit" or "George"? "The stranger" or "Bob"? Yeah, that's what we thought. And that's exactly why Natalie Babbitt didn't give him a name. Don't believe us? Hear it from the horse's mouth.