N i n j a s

Zabuza
A mercenary ninja and the first opponent we see battling Kakashi. He was hired by Gatou to kill the bridgebuilder that Kakashi and his team were ordered to protect. Zabuza and Kakashi have a good fight but in the end he's beaten by Kakashi twice (but never lost completely). I guess Zabuza and Kakashi do have a respect between them, like powerful people respect eachother.
 
Haku
Always by Zabuza's side, Haku is a genius child with advanced blood. Kakashi is in awe of Haku because of his talent at such a young age (for example, when Haku did a ninjutsu using just one hand). Haku was rescued by Zabuza when he was homeless on the streets and so he saves Zabuza's life by throwing his body in front of Kakashi when he was about to land the killing blow.
 
Orochimaru
The psycho to end all psychos. He's probably the first (if not only ninja) that Kakashi showed an outward fear towards. Orochimaru's one of the three legendary leaf ninjas who's even more powerful than the Hokage. Orochimaru even noticed that Kakashi had a new Sharingan eye. Kakashi was going to fight Orochimaru to end his pursuit of Sasuke, but even Kakashi realized he wouldn't be able to defeat him.
 
Kabuto
A guy who failed the chunnin exam so many times...but is secretly Orochimaru's henchman. It's not clear whether or not his loyalty to Orochimaru is really what it seems. Kabuto isn't a jounin but he's at the same level as Kakashi...which makes him a fierce opponent (he even found Kakashi's weakness with his blind spot. Even Kakashi's confused what's going on in Kabuto's head.
 
Ebisu
A tutor for the elite ninjas, Naruto first met Ebisu in the beginning when he befriended the Hokage's grandson. Kakashi personally asked Ebisu to train Naruto basic Chakra controlling skills (which Naruto wasn't very happy about and kept calling Ebisu a closet pervert). Ebisu has deep respect for Kakashi or else he wouldn't have accepted tutoring Naruto. Kakashi on the other hand wonders why Ebisu's a closet pervert...
 
Jiraiya
One of the three legendary leaf ninjas along with Orochimaru, Jiraiya's the one who ended up tutoring Naruto after Ebisu was knocked out by accident when he caught Jiraiya peeping into the women's bath. Like the other legendary leafs, Kakashi has respect for him (and Kakashi probably would have more respect if he found out Jiraiya's the author of the romance series "Come Come Paradise" he's always reading).
 
???
Her name is not yet revealed in the manga, but she's definitely somebody who's obviously close to Kakashi. She's part of an Anbu (assassin) team and we first see her encounter with Kakashi on the rainy day where the heroes monument was and she refers to Kakashi as "Kakashi-sempai" (a term for a respected upperclassman). She also seems to know Kakashi's dead friend...hope to see more of her soon!
 
Uchiha Itachi
If you don't recognize the name, then you definitely gotta pay closer attention to the series! Itachi is Sasuke's older brother...the SAME man that Sasuke has vowed to kill. Like Kakashi, Itachi has a Sharingan eye...in fact, two! Kakashi fought Itachi in order to protect Asuma and Kurenai, but ended up losing to him after Itachi used his Tsukiyomi technique with his eyes.
 
Tsunade
The female of the legendary three ninjas, Tsunade is an incredible healer and brings both Kakashi and Sasuke out of their comas after their fight with Itachi. It's hinted that Tsunade does know Kakashi on a personal level, but evidentally the last time she saw him was when he was considerably younger.