One shot of Princess Lucia that was published the year before it was serialised. Serves more as a prequel to the series than as a standalone version.

Ogata Natsuki is the best player on the Kakuchi Junior High Basketball Team. Which is not saying much because they couldn't even get through the 3rd round of the Prefectural Tournament from freshman year until the end of Junior High. For High School, his friends convince him to try out for the Number One High School Basketball team, Fujiwara Academy. So he goes to live with his aunt who runs a hot springs resort to go and try out for the best. Fujiwara Academy.
The thing is...he's small. So people look down on him and he has trouble convincing people he's any good. Through one thing or the other, he ends up with Tachibana Mitsuki watching her school, St. Mariannu High go up against Fujiwara Academy in a practice game. But the thing is, St. Mariannu is very weak. Weaker than even his former school. Fujiwara is just toying with them and now they don't even want to finish the game. What will happen now? Will Natsuki ever be able to achieve his dreams of being High School Basketball's Number 1? And will it be with Fujiwara?
(Source: Selene Scans)

4-koma manga specials included in volumes of Kimi no Iru Machi

Based on DS Dating Sim "Love Plus"

Aikawa Wataru's father is being transferred overseas. Seeing this as a perfect opportunity, Wataru decides to stay behind in Japan and live on his own. Even though he's a new transfer student, his teacher convinces him to become a Library Representative. A fellow Library Rep tells him they’re missing a member and asks Wataru to find her. Not knowing how she looks like, he wanders the school trying to find her only to notice two girls gossiping about her negatively. It turns out the person he's looking for isn't well liked, because of her bad personality.

Her name is Kobayakawa Rinko, but what's the source of her bad attitude?

(Manaka also makes an appearance)

Eba Yuzuki has mysteriously decided to go to high school in the countryside. But despite Kirishima Haruto's objections, she is living in his home. Now he has to put up with a freeloading city girl and even worse, make sure Kanzaki Nanami, the girl he likes, doesn't get the wrong idea!

The story’s setting is based on the author, Seo Kouji’s hometown.

Kenzaki Ryuunosuke is a new editor for Weekly Shounen Magazine, but his love for an unpopular series gets him no love with his coworkers. He meets Takanashi Tsubasa, an aspiring mangaka, and seeks to prove himself by making her the number one manga artist in Japan.

While Japan was at war with the world, their romance has just started...

The last one-shot manga in Seo Kouji's Love Letter tankoubon. The other one-shots are Love Letter part 1, part 2 and Half & Half.

From Del Rey:
Yamato is ready for a fresh start. So when his aunt invites him to stay rent-free in her big-city boarding house in hustling, bustling Tokyo, Yamato jumps at the chance. There's just one teensy-weensy catch: It's an all-girl housing complex and spa! Things get even more nerve-racking when Yamato meets his neighbor Suzuka, a beautiful track-and-field star. She's not just the cutest girl Yamato's ever met, she's also the coolest, the smartest, and the most intimidating. Can an ordinary guy like Yamato ever hope to win over a girl like Suzuka?

HALF & HALF(1996)
A debut manga work by Seo Kouji
HALF & HALF (2007)
Nagakawa Shinichi and Sanada Yuuki both get involved in an accident and die together, but they are given a chance to live again. A voice lets them survive for seven more days. However, that after the time limit, one of two must die again.
Also, their lives will be shared, including emotions (guilt) and physical sensations (pain). They are told not to leave the other's side, else both of them die. Now, with each of them wanting to save his/her own life, how will the next seven days of non-separation be like? Will they get along?
Half & half (2012)
Serialized in Bessatsu Shonen Magazine

The story follows the life of Yuu Haruna, who recently moved into Tokyo with his sisters after their father is forced to transfer overseas on work.

On his way to buy dinner while looking at his Twitter account, a high school girl suddenly crashes into him. Thinking he was taking upskirt pictures of her, the girl takes Yuu's phone, breaks it, and slaps him before leaving Yuu lying on the ground. As it turns out, this girl—Fuuka Akitsuki—also goes to the school Yuu is transferring to.

Unlike most people, Fuuka doesn't own a cellphone; she even listens to music using a CD player. Eventually these two become closer, and decide to form a band with their friends and enter the professional world of music. With Fuuka around, what will now become of Yuu's new life in Tokyo?

(Source: MAL Rewrite)