Once upon a time...
(based on vol 9 of the manga but no spoilers ;)
"Out of my 32 students I call only recall 15...(1)
Fuura-san, Okusa-san, Otonashi-san, Kaga-san, Kitsu-san
Kimura-san, Kobushi-san, Komori-san, Mataro-san, Tsunetsuki-san,
Hito-san, Fujiyoshi-san, Mitama-san, Kotonon-san and Kudo-san."
With that everyone in the class sweatdropped.
"Sensei. You only recall the girls, don't you?" said Kafuka
"That's strange and kinda disturbing" added Abiru
Of course each one of those girls had their special trait. Fuura-san was the optimistic, Osuka-san was married, Otonashi-san was a mail abuser, Kaga-san was always apologizing, Kitsu-san was a perfectionist, Kimura-san had double personalities, Kobushi-san was always injured, Komori-san was a social reclute, Maria-san was an illegal poor inmigrant, Tsunetsuki was stalking him, Hito-san was the most normal ordinary girl in the planet, Fujiyoshi was addicted to yaoi, Mitama was evil and looked evil, Kotonon was an ugly net idol and Kudo-san was... wait... wasn't Kudo-san a guy?(2)
"Sensei. I'm a guy" added Jun Kudo from behind still holding his book
"So, Kudo-kun is the only guy sensei can remember, right?" added Fuura from far behind. While Kagero Usui, the nearly bald guy made gestures to get himself noticed. (3)
"But it can't be since Kudo-kun has no special trait. Everyone I recall has something unique about them, otherwise they aren't worth remembering in this world"
"Sensei..."
"YOU MUST SEARCH FOR SOMETHING THAT MAKES YOU UNIQUE! Otherwise you will be overshadowed.
No one would remember the Pisa Tower if it was straight like any other tower. No one would recall Napoleon if he hadn't conquered so much. No one would recall the Roman empire if it hadn't occupied most of Europe!"
"Nee, Sensei..."
"So, Kudo-san. I remember you were good at something but don't remember what was it..."
"Kudo-san can tell you a story if you tell him just a single word. No more stuff. (4)" closed Chiri-chan
"I am can cook too..." added sarcastically Kudo-san without taking his eyes from his book.
"Telling stories is not a special talent. It can only be used for a school feast or for amusement. It is the same as being overweight. It is handy if you are a circus star. Or being very short. You can debut in the Guiness Records and -" Itoshiki Nozomu, the despair teacher got carried away with his senseless examples. "- so it is useless in real life, but why do I remember you?"
"Maybe he's normal after all" added Nami
"No. There's no more normal and ordinary person than Nami-san"
"That's cruel" cried Nami in TT tears
The despair teacher looked everywhere
"I'M DESPERATE! Remembering someone with no special trait has left me desperate!"
Kudo-kun just smiled. He decided that when he grows up to be a famous novelist he'd make a book about his unusual sensei. He resumed reading his peaceful mode.
Author notes:
(1) I've added 15 which includes the characters in the manga version as well not shown in the anime, such as the housewife.
(2) In the manga and in the first episode of Zoku Zetsubou Sensei (season 2) we can see the 10 main students recalled by the teacher (15:20) being the others covered by coffee stains. These are the 9 main girls and Jun Kudo, being a total of ten. The 9 girls have comments written on about their weird stuff such as Matoi being described as dangerous or Kimura as double personalities. Jun Kudo remains without a comment
(3) Nozomu can be able to recall up to 16 students whenever the baldness of Usui-san shows. Otherwise the only guy he can recall is Jun Kudo
(4) Jun Kudo hasn't been shown to have any other special traits rather than story telling which is not a personality trait as the other main characters, but rather an ability