Harry Potter and the Chamber of Secrets
We get into the specifics of Tom Riddle's diary in "Symbolism, Imagery, Allegory." Here, we're just going to talk about Tom Riddle as a character. He is essential to the overall arc of the novels because, as he says, "Voldemort […] is my past, present, and future" (17.69). Riddle's full name (Tom Marvolo Riddle) is an anagram for "I Am Lord Voldemort." For the whole year, Ginny has been corresponding in a diary with the young Voldemort. No wonder she's been looking pale and sickly for so much of the time.
One of the things we love about Book 2 is that we get to see how Voldemort works without immediately recognizing that it's Voldemort, since sixteen-year-old Tom Riddle is a long way from the snaky-faced creature we saw peering out of the back of Professor Quirrell's head in Book 1. Tom Riddle is handsome and appealing; as he tells Harry, "If I say it myself, Harry, I've always been able to charm the people I needed" (17.45). Ginny writes to Tom Riddle, feeling that he is "like having a friend [she] can carry around in [her] pocket" (17.43). Even Harry falls for it at first, believing Riddle's memories without imagining that Riddle might have his own motives for setting up Hagrid to take the blame for the Chamber of Secrets. As Professor Dumbledore concludes, "Older and wiser wizards than [Ginny] have been hoodwinked by Lord Voldemort" (18.23). He's good at gaining trust to manipulate people.
Still, the second aspect of Tom Riddle's personality that clearly foreshadows his transformation into Voldemort is his utter coldness. He has absolutely no shred of sympathy for the people around him. When he discusses possessing Ginny or framing Hagrid, he talks about their lives with so much disdain that it's almost violent: "So I made Ginny write her own farewell on the wall and come down here to wait. She struggled and cried and became very boring" (17.64). Riddle is talking about an eleven-year-old girl trying to save her own life, and the only adjective he can muster is "boring." Even at sixteen, Riddle has no feeling for others, which is what allows him to become the creature he is at the time that he murders Harry's parents.
In fact, J.K. Rowling has emphasized precisely this aspect of Voldemort's personality in interviews. When asked if Voldemort had ever loved anyone, she replied: "No, never. […] If he had, he couldn't possibly be what he is" (source).
One other hugely important detail of Voldemort's character appears in Chamber of Secrets. Riddle tells Harry that his father was a "foul, common Muggle, who abandoned [Tom Riddle] even before [he] was born, just because he found out his wife was a witch" (17.72). In other words, despite all of the pureblood crap that his followers spout, Voldemort himself is half-Muggle. Voldemort's origins prove that he may hate Muggles (in fact, he definitely hates Muggles), but what he wants more than this pureblood stuff is his own personal power. If he needs to manipulate purebloods to get it, that's what he'll do. Blood is certainly more important to a man like Lucius Malfoy than to Voldemort.