Necessary Evil by Grimbald
The Shrinking Swordsman
Neutrino was an avid environmentalist named Bryan Gibson who traveled to Brazil as part of a group campaigning to stop the deforestation of the rain forest. When his (rather naive) group attempted to confront some loggers, they were badly beaten and nearly killed. Most of the group made it back to their camp, but Bryan, delirious, became lost in the jungle and eventually collapsed into unconsciousness.
When Bryan awoke, he found himself in a secret civilization of 6" tall yellow-skinned humanoid aliens called Morlaidhans. Their princess Laethwyn had found him, shrunk him down, and brought him to the city. Bryan, even more determined to stop the loggers in order to save this civilization (and the woman he falls in love with), rapidly learns their warrior ways. Armed with his sword and the ability to adjust his size from 6" to 6’, he manages to stop the loggers (with the help of the tiny warriors and his friends from the environmental group.) Unfortunately, though the city is saved, Laethwyn is killed in the battle.
In her honor, Bryan swears to continue his quest to stop the pillaging of the Earth by greedy corporations, and takes on the guise of Neutrino. For the few years before the alien invasion, Neutrino was a vigilante who skated the line between hero and villain—sometimes cooperating with other heroes to face some bigger threat, sometimes wanted by the authorities for his violent tactics in stopping polluters. When the V’Sori invaded, he was deep in the Amazon rainforest visiting the Morlaidhans, and so survived the attacks. When he returned to civilization, he swore to protect the Earth as best he could from this new enemy.