Summary:
A prelude of sorts to X/1999, Tokyo Babylon follows Subaru and his bubbly sister Hokuto, mediums who protect Tokyo from restless spirits. They meet up with the mysterious Seishirou who, as it turns out, has a violent past...and a violent future. Two of these characters from Tokyo Babylon play an important role in X/1999.
I've never read this series, but it has a devoted following of admirers, sure to grow as it is now being released in English by Tokyopop.
A two-part OAV was created and is available in English (see links below).

Best For:
Ages 13 and up.

Contains:
Mystery, Occult, Drama, Violence, Shonen-ai (boy-boy romantic elements)

Read it: You can buy the English-translated manga, Vol. #1, Vol. #2, Vol. #3.

Watch it: You can buy the Tokyo Babylon OAV DVD.