With regards to religious belief or religious affiliation, Poison Ivy can best be described as a fundamentalist Environmentalist.
Below: Poison Ivy explains her fundamentalist Environmentalist goals to Batman. She is willing to murder humans for the sake of her radical Environmentalist agenda. [Source: Batman #651, DC Comics (May 2006), page 17; reprinted in Batman: Face the Face, DC Comics (2006), page 46; written by James Robinson, pencilled by Don Kramer, inked by Keith Champagne.]
Below: Rather than see her beloved plants killed by a defoliant Batman has had Robin inject into the building's sprinkler system, Poison Ivy gives herself up. She only "wanted to the do the right thing," she proclaims. From Batman #651, page 22:
Text from scene above, from Batman #651, DC Comics (May 2006), pages 14-22; reprinted in Batman: Face the Face, DC Comics (2006), pages 41-49; written by James Robinson, pencilled by Don Kramer, inked by Keith Champagne:Batman: Hello, Pamela.
Poison Ivy (whose real name is Pamela Lillian Isley) is one of Batman's most important recurring enemies in the various DC Comics comic book series that feature the Dark Knight and his Gotham-based allies. Poison Ivy has been featured in movies, live action television and animation. She is one of Batman's most widely recognized foes.
Nothing is really known about Poison Ivy's religious background or upbringing prior to her becoming an enemy of Batman. She is not known to have identified herself as an adherent of any organized traditional religious group or denomination.
What is clear about Poison Ivy is that she has a fanatical obsession with plants and radical environmentalism. Her ambitions and goals far exceed that of most traditional and mainstream Environmentalist groups such as Greenpeace or the Nature Conservancy. Her willingness to kill humans for the sake of her beloved plants marks her as even more extreme than real-world fundamentalist Environmentalist groups such as the Earth Liberation Front (ELF).
With regards to her principle motivations and beliefs, Poison Ivy is in some ways similar to another major Batman villain who is a fundamentalists Environmentalist: Ra's Al Ghul. But whereas Ra's Al Ghul's goals are often complex and never focus on eliminating all human life (just most of it), Poison Ivy appears to have an almost monomaniacal obsession with plants. Fortunately for Batman and civilized society, Poison Ivy is considerably less organized and a far less effective killer than Ra's Al Ghul.
Poison Ivy strives to consciously identify with plants more than with humans, although she is herself human. Her disdain for humans may even exceeds that of Ra's Al Ghul, who still regards himself as human, although superior to most other humans. Fellow Batman villain Harley Quinn is Poison Ivy's only regular human friend.
Despite the fact that Ra's Al Ghul and Poison Ivy have both been classified as "fundamentalist Environmentalists," one should not conclude from this that they somehow share the same "religious affiliation." There is certainly no umbrella fundamentalist Environmentalist organization to which they both belong. it is reasonable to conclude that they are aware of organized radical Environmentalist groups (such as ELF), but they probably regard such organizations as ineffective and insufficiently radical. Despite some similarities in their goals, Poison Ivy is not known to be an ally of Ra's Al Ghul.
Poison Ivy has joined criminal organizations such as the Injustice Gang and the Secret Society of Super Villains. She has also temporarily allied herself with other villains. These alliances have always been for the sake of expediency as a means of furthering her own goals or vanquishing her enemies, and have not been based on any shared philosophy or religious beliefs.
From "Poison Ivy" article on Wikipedia.org (http://en.wikipedia.org/wiki/Poison_Ivy; viewed 19 April 2007):
Poison Ivy (Pamela Lillian Isley) is a fictional character, a DC Comics super-villain who is primarily an enemy of Batman. Created by Robert Kanigher and Sheldon Moldoff, she first appeared in Batman #181 in (June 1966).
In the series Gotham Girls, Poison Ivy deems herself as one of "the world's most prominent eco-terrorists." She is obsessed with plants, botany and environmentalism. She utilizes toxins from plants and her own bloodstream for her criminal activities, which are usually aimed at protecting the natural environment...
Poison Ivy did not initially catch on as a character, and was not heard of again until the rise of feminism brought the need for a greater number of more independent female villains in the series. She was also used to replace the increasingly sympathetic Catwoman as a clearly antagonistic female super-villain for Batman, and then made further appearances in the Batman comic book series and in Suicide Squad...
In some circles, much has said about Poison Ivy's sexuality, with speculation that she may be a GLBT person or a lesbian. She is not known have explicitly identified herself with such classifications. To do is an over-simplification of her character and may be inaccurate.
In point of fact, Poison Ivy's romantic relationships and infatuations have nearly always been with men. Suppositions about Poison Ivy's sexuality largely stem from the way she has, in recent years, been illustrated alongside her frequent partner-in-crime, Harley Quinn.
A frequent modus operandi utilized by Poison Ivy is to use plant-based chemical concoctions to cause men to fall in love with her or fall under her spell so that she can manipulate them. She has done this to Batman and even Superman. She has at times pronounced her love for her victims, including Batman. Her chemical control over individuals is sometimes delivered through a kiss.
None of this indicates a normative of societally accepted category of sexual preference. Her behavior is indicative of a strongly aberrant psychosexual persona. She might be classified as omni-predatorial in her sexuality. That is, she is indiscriminate in the gender of the people she uses and abuses through sexual and emotional manipulation toward villainous and murderous ends. Poison Ivy's relationship with Harley Quinn (whether mere platonic friendship or more), appear to be an exception to this pattern of behavior, and appears to be focused on Harley Quinn as an individual, rather than a fixation on women generally.
Regardless of whether Poison Ivy is best characterized as GLBT, LGBT, lesbian, bisexual, heterosexual or simply psychosexually deviant, all of these elements of her behavior and character are secondary or subservient to her primary motivation/ultimate concern/religion: her obsession with plants and radical environmentalism. In no way does Poison Ivy identify herself primarily based on any aspect of her sexuality. Furthermore, Poison Ivy's sexuality is entirely tangential and in no way in conflict with her fundamentalist Environmentalist beliefs. For this reason, there is no "GLBT" notation next to Poison Ivy's listing of her religious affiliation/belief in the main table, such as there is with characters whose GLBT lifestyle conflicts with their known religious affiliation (as is the case with GLBT Catholics such as Batman ally Renee Montoya).