Wading into the original L.J. Smith books after watching the TV show is a wild trip, because the character foundations are both familiar and totally different. The absolute core is Elena Gilbert, Stefan Salvatore, and Damon Salvatore, that love triangle is the engine. But book Elena is a regal, popular blonde with ice-princess vibes, not the vulnerable sweetheart from the show. Stefan is softer, more haunted by his past, and Damon is less a sympathetic bad boy and more legitimately cruel and predatory in the early books—their dynamic is way more black-and-white initially.
Then you've got Bonnie McCullough, who's a fiery redhead with strong psychic powers, and Meredith Sulez, who's calm, logical, and a gifted fighter. Their friendship with Elena is crucial. Matt Honeycutt is there, pining for Elena, and Caroline Forbes is the jealous rival. The Salvatore brothers' backstory with Katherine von Swartzschild is a huge plot driver, though she's less of a direct presence. It's funny, the books have this very 90s YA gothic feel, so the characters operate with a different set of rules and motivations than the later TV adaptation built for drama.