Takahashi Rumiko is clearly a veteran with her career spanning several decades. Not only has she touched upon both shonen and seinen, the tone of her works can range from light-heartedness (Kyoukai no Rinne) to the macabre (Mermaid Saga).
Please note that the following list is not comprehensive of everything she's done! The summaries are from Baka-updates, which got theirs from VIZ.
Urusei Yatsura (うる星やつら)
Genre: comedy, romance, science fiction Magazine Serialized In: Weekly Shonen Sunday (1978 - 1987) Status in Japan: completed; 34 volumes Target Age Group: teens
What would you do if a tiger skin bikini-clad alien followed you to school every day? Poor Ataru's life has never returned to normal since volatile extraterrestrial princess Lum fell for him. Now an excruciatingly wealthy and genteel rival, Mendo, adds to Ataru's torment. Not to mention an unending stream of extraordinary classroom visitors and educational materials including incompetent cherry blossom spirits, a legendary nightmare-eating tapir, and a fourth-dimensional camera that breeches alternate realities.
Maison Ikkoku (めぞん一刻)
Genre: comedy, romance Magazine Serialized In: Bic Comic Spirits (1980 – 1987) Status in Japan: completed; 15 volumes Target Age Group: mature
Travel into Japan's nuttiest apartment house and meet its volatile inhabitants: Kyoko, the beautiful and mysterious new apartment manager; Yusaku, the exam-addled college student; Mrs. Ichinose, the drunken gossip; Kentaro, her bratty son; Akemi, the boozy bar hostess; and the mooching and peeping Mr. Yotsuya.
Ranma ½ (らんま½)
Genre: martial arts, romance, comedy, urban fantasy Magazine Serialized In: Weekly Shonen Sunday (1987 - 1996) Status in Japan: completed; 38 volumes Target Age Group: teen
Being a teenage martial artist isn't easy, especially for Ranma Saotome, who went through a major transformation on a training mission with his father. After an accidental dunk into a legendary cursed spring in China, Ranma now changes into a girl every time he's splashed with cold water. That would be enough to complicate anyone's life, even without the arranged fiancée who doesn't like him (or says she doesn't) and the constant stream of rivals and suitors for both his male and female forms. What's a half-guy, half-girl to do?
Genre: adventure, historical fantasy, romance, drama, supernatural Magazine Serialized In: Weekly Shonen Sunday (1996 - 2008) Status in Japan: completed; 56 volumes Target Age Group: teen
Kagome is a modern Japanese high school girl. Never the type to believe in myths and legends, her world view dramatically changes when, one day, she's pulled out of her own time and into another! There, in Japan's ancient past, Kagome discovers more than a few of those dusty old legends are true, and that her destiny is linked to one legendary creature in particular--the dog-like half-demon called Inuyasha! That same trick of fate also ties them both to the Shikon Jewel, or "Jewel of Four Souls." But demons beware...the smallest shard of the Shikon Jewel can give the user unimaginable power.
Kyoukai no Rinne (境界のRINNE)
Genre: supernatural, school life, comedy Magazine Serialized In: Weekly Shonen Sunday (2008 - ) Status in Japan: on-going Target Age Group: teen
As a child Sakura Mamiya mysteriously disappeared in the woods behind her grandma's home. She returned whole and healthy, but since then she has had the power to see ghosts. Now a teenager, she just wishes the ghosts would leave her alone! At school, the desk next to Sakura’s has been empty since the start of the school year, then one day her always-absent classmate shows up, and he's far more than what he seems!
Mermaid Saga (人魚シリーズ )
Genre: action, romance, horror Magazine Serialized In: Weekly Shonen Sunday (1984 - 1994) Status in Japan: completed; 4 volumes Target Age Group: teen
Legend has it that if a human eats a mermaid's flesh, they will be granted eternal life. But is that a blessing or a curse? Immortal lovers Yuta and Mana travel through Japan and the centuries longing for a normal life. On their journey, they meet others who have consumed the flesh of a mermaid in hope of becoming immortal. But the powerful substance affects each person differently. Yuta and Mana are two of the lucky ones. Others are fated to die or transform into a savage monster, a "lost soul." Mermaid Saga contains three of Takahashi's uniquely macabre mermaid tales, A Mermaid Never Smiles, The Village of the Fighting Fish, and Mermaid Forest. Each revolves around Yuta, who wanders for hundreds of years, seeking a mermaid to cure him of his immortality.
One-pound Gospel (1ポンドの福音)
Genre: sports, romance, comedy Magazine Serialized In: Weekly Young Sunday (1987 - 2007) Status in Japan: completed; 4 volumes Target Age Group: mature
Kosaku Hatanaka is the first pro boxer ever produced by Mukaida's Gym. He's also the bane of his coach's existence. Although he's constantly waging war against his appetite, Kosaku never met a meal he didn't like! Naturally, this plays havoc with his weight class. Junior bantamweight, bantamweight, junior featherweight... every time he moves up a class, his coach's hopes plummet. Isn't gluttony a sin?
Sister Angela is a young novice nun who's dedicated, passionate, and convinced that we should give peace a chance. Although she refuses to acknowledge it, it's obvious that Kosaku comes to St. Mary's for more than confession! He comes to see the smile of the beautiful sister, of course! Can her faith redeem the gluttonous appetite of the tempted lamb astride the scale?