Onyx Adams (typically known simply as "Onyx") is a Buddhist superheroine who is part of the "Batman family" of characters. Onyx trained (and has often lived) at the same Buddhist monastery that produced the second Green Arrow: Conner Hawke.
Conner Hawke has always been overtly Buddhist and often identifies himself as a Buddhist by name, but Onyx is not known to do the same. The religious affiliation of Onyx can be identified as Zen Buddhism - the same sect that that the second Green Arrow devoutly subscribes to. But Onyx herself does not appear to have as strong a Buddhist self-identity as Conner Hawke, despite the fact that she received very similar religious training.
Above: Capsule biography for Onyx, from Batman: Under the Hood trade paperback, published by DC Comics (2005), page 6:
Onyx was schooled in the Sancruary, a monastery outside Star City. Pursued by an unknown agent who wanted to kill her, Onyx was admitted to the all-male Sanctuary by the Master, who trained her in martial arts and gave her a new identity. Upon the Master's death, Onyx sought out Green Arrow, another student of the Sanctuary, to protect the monastery from a takeover by one of the Master's more ambitious proteges, a man named Lars. Onyx stayed at the Sanctuary, guarding the key to the Master's mysterious Book of Ages. She has yet to discover the identity of the man, or woman, who pursued her to the doors of the Sanctuary.
From "Onyx (comics)" webpage on Wikipedia.org website (http://en.wikipedia.org/wiki/Onyx_(comics); viewed 16 June 2007):
Onyx is a DC Comics fictional character. She is a superhero based in Gotham City and an ally of Batman. She is highly trained in martial arts and serves as a member of the League of Assassins before reforming and becoming a vigilante.
When Onyx leaves the League she chooses a life in an Ashram Monastery, an order to which Green Arrow once belonged. The Ashram's Exalted Master is later killed by the renegade monk Lars, who intends to unravel the "Wisdom Key" that unlocks the powerful Book of Ages. Onyx sought Green Arrow's help to defeat the villain. In Ashram, Green Arrow and Onyx were defeated by Lars' minions, and the monk takes the Wisdom Key from Onyx. Upon opening the Book of Ages, however, Lars is vaporized.
Later, Onyx appears in Star City to battle a villain named Barricade. Barricade turns out to be Lars, who was revived by his underlings. The spell that had apparently killed him was reversed only as long as he remained in contact with the Book of Ages, but he can make the reversal permanent with possession of the Wisdom Key. In a battle with Onyx, Green Arrow, and Black Canary, Barricade again meets defeat and is reduced to a heap of bones.
Following this incident, Onyx is not to be seen for years. She briefly returns as part of Wonder Woman's team of super-heroines who battle Circe's minions.
After that, Onyx returns to monastery life but is called upon by Batman to serve as a bodyguard for Orpheus, and she works alongside Orpheus until his death. Onyx's notable appearances include the "Under the Hood" arc, in which she battles the new Red Hood, and the "War Games" crossover. Onyx is the only independent vigilante Batman allows to operate freely in Gotham City.