Koyomi Araragi is a third-year high school student who survived a vampire attack during spring break and barely regained his human life as a boy with remnants of vampiric traits. He can see in the dark, has red eyes when he gets angry, and heals incredibly fast. Despite no longer being affected by morning sunlight, Koyomi remains scared of it due to past experiences.
Koyomi is laid-back and his only friend at school is Tsubasa Hanekawa. Although he's a poor student, except for math, he has shown knowledge about anime. His attraction towards other girls is noted by Hitagi, but Koyomi turns them down due to loyalty to Hitagi. Other characters speculate that Koyomi might be a lolicon, particularly Mayoi and Hitagi, with Hitagi thinking Nadeko is a "threat" to her.
Koyomi's personality is portrayed as noble, which attracts him to girls like Hitagi. He's willing to help others at the cost of his own life, making female characters attached to him. Koyomi feels most comfortable talking to Mayoi, Shinobu and Yotsugi about his problems.
Koyomi thinks that his family relationship is bad, while Karen and Tsukihi think otherwise. His parents were police officers. The novels reveal more about Koyomi's personality, showing him as perverted and intelligent, but the anime may not convey this fully. Prior to Kizumonogatari and Bakemonogatari, Koyomi avoided having friends and seemed to have lost faith in people and himself.