Viktor Krum is the champion from the Durmstrang Wizarding School—and, apparently, an amazing Quidditch player. Ron is all googly-eyed about him when they get to watch him play at the Quidditch World Cup.
Ron is less into Krum, though, when he shows up at Hogwarts to compete in the Triwizard Tournament. Don't get the wrong idea, though: it's not about the fact that Krum is competing against his fellow Hogwartians Harry and Cedric. Actually, it's because Krum takes a shine to Hermione.
Most of what we learn about Krum comes from how people act toward him, or what people say. Because Krum himself? He doesn't really talk that much. Hermione pretty much sums it up when she's talking to Harry about her budding romance:
HERMIONE: Actually, we don't really talk at all. Viktor's more of a physical being.
Yowza. But it's true: while he seems very solicitous and gentle in his gestures and attitude with Hermione, we just don't really hear him talk all that much.
The only time he doesn't seem super nice is when he ends up bewitched in the maze and seems ready to go after Harry. But given that he was under a spell, we think we can forgive that one.