Main Character list:
Chihaya - (plus-checker) short bishonen with black hair and dark eyes (black in the manga, anywhere from reddish to violet in the anime), black wings.  Very naive, usually optimistic, but also very sensitive and prone to depression.

Kagetsuya - (minus-checker) Chihaya's partner, tall and beautiful with blond hair and blue eyes, white wings.  Cynical and pessimistic, often impatient.  The only time you see any real vulnerability in him is when it involves Chihaya.

Dr. Ashino - A not-bad-looking nutcase, has brown hair and wears glasses.  Creepier/more menacing in the subtitled version, loonier in the dub - but a nutcase, either way. ^_~

Taki and Takako - victims of the mad Doctor, who appear in the first episode (The Beginning of the End).

Blair and Cliff - two individuals whose lives are touched by angels in the second episode (Fallen Angel).

Messiah - a tormented soul (oh, the angst!) who makes his appearance in episodes three and four (Angelic Destroyer and Final Battle).

Short synopsis of the story theme:
The angels in Eden have been watching over Earth and its inhabitants (the Earthians) for years. They are trying to determine whether humans are a positive race or a negative one.  The angels send representatives to Earth in pairs to gather information; each pair is comprised of a plus-checker and a minus-checker.  It is the duty of the investigators to record the positive or negative aspects of their experiences, and report to Eden.  If the number of negatives outweighs the positives, the angels will destroy the Earth.

The anime follows part of the adventures of two angels, beginning with their experiences in Shinjuku and their first run-in with a mad scientist and his creations.  Lamentably, there are only 4 episodes in the series.

Potential slash couples:
Kagetsuya/Chihaya:  This one is canon; they have an all-too-short scene in Angelic Destroyer that makes it clear. *^_~*  The pairing is also canon within the manga context.  I love these two together; there is a definite sweetness there, as well as some beautiful angst. :)

Chihaya/Kagetsuya:  Well, it could happen... but it seems more likely to me that Kagetsuya would prefer to be seme most of the time, and it also seems likely that Chihaya would prefer to be uke most of the time.

Kagetsuya/anyone other than Chihaya:  Really not likely, unless it's an AU (or an original character being brought in for that purpose).

Dr. Ashino/Chihaya:  Scary, but you could make a good argument for it.  It'd pretty much have to be NCS.  *shudder*

Chihaya/Dr. Ashino:  No way.  Not likely to happen, unless it turns out one of them is having a psychedelic dream.

Dr. Ashino/Taki:  Again, scary but you could make a good case for it.

Taki/anyone else:  Probably not; mostly for lack of opportunity.

Cliff/anyone but Blair:  Sorry, but no.  Lack of opportunity and lack of interest. ^_~  But it would be really pretty.

Messiah/Chihaya:  Could make a very good argument for this one, though how they'd find the time is beyond me.

Chihaya/Messiah:  If Chihaya were going to be a seme... this is a possibility.  Messiah is so needy, and at that point Chihaya would know what's what. ^_~  But again, there doesn't seem to be much opportunity.

Dr. Ashino/Messiah:  Sadly, this is possible. *shudder*

Suggested URL's:
"Earthian, by Yun Kouga"
http://www2.hawaii.edu/~vickyl/earthian.htm
has a fanfiction section, which includes almost every Earthian story written.  I think only one of them is non-yaoi ("New Angel" by Kelly Min). ^_~  The site also has image galleries, information on the series, and links to most of the Earthian sites out there. :)