Karma measures your character's "goodness" in Cythera. According to the documentation, actions like lying, stealing, cheating, and killing will lower your karma, but in the actual game, very few things affect your karma. While you can't check the value directly, it can be estimated from the shade of gray your name is in the dialogue window. 100 (white) is the highest, and 0 (black) is lowest. You begin the game with a karma value of 55.

Just as karma isn't affected by much, it doesn't really have much impact on your character either. Mostly, the seldane will not speak with you and Alaric will not heal you if your karma is too low, but there don't appear to be any other times it makes a difference. BreadWorldMercy453 constructed the following list of things that change your karma value and the amount of change.

Action Effect
Curing the plague +10
Finding Thersite's ring +5
Freeing Maayti +10
Giving oboloi to drunken beggars +1 per 10 oboloi given at a time
Killing a flying bird +1
Killing a generic NPC +1 (Probably a bug)
Killing a monster (golem, polyp, hydra, tentacle, Scylla, demon, skeleton, undead, lich) +4
Killing a ruffian +4
Killing Magpie -10
Killing someone with the appearance of the hero or heroine -10
Putting flowers on Andra's grave +5
Rescuing Ariadne +10
Returning Stentor's net +5
Stealing if it stains your soul -1

For the sake of reference, I am keeping as many of BreadWorldMercy453's notes as possible. So here is her original, unaltered list of activities that she tried that did not affect karma:

Killing any kind of wild animal except for the ones listed above.
Killing goats or chickens, even when the NPCs get mad & attack you.
Completing any to-do list task except for the six listed above.
A follower killing anyone/anything, even if you've specifically directed them to do so.
Stealing something if the deeds don't stain your soul (taking food, moving an item before taking it, training in Thievery, etc.)
Sleeping in somebody else's bed.
Snatching Emesa's flatbread off of the stove & eating it before she can take it back.
Taking the deep-sea kelp out of Glaucus' must vat.
Refusing to pay Aethon's bar tabs.
Joining the Ratcatchers' Guild, and training in their shady skills.
Gambling, even if you've trained in Gambling.
Giving 9 oboloi or less to Eumelus or Ennomus.
Nutrienting hungry NPCs.
Breaking other people's mirrors, crates, or chests.
Accusing the Comana brothers of lying, whether or not you caught them with Ascertainment.
Buying things from NPCs.
Offending merchants by haggling too much.
Re-uniting Philinus & Tlepolemus.
Being a messenger for Hebe, Antenor, and Crito.
Listening to Neoptolemus' boring story, learning his skills.
Shaking down an NPC, taking their stuff.
Putting together the Crolna.
Neutralizing the Crolna.
Lying to NPCs (there's not a whole lot of opportunity to do this, but I tried: telling Emesa I was hungry when I wasn't, telling Anisa I'd learned about Alaric's mother when I hadn't, telling Sacas I'd learned something useful from Antiphus when I hadn't, telling Hebe I had flax to sell when I didn't, telling Apis I'd negotiated her wine contract, telling Neoptolemus that Alaric told me his name, and telling Jhiaxus that I'd honored Jinrai when I hadn't).

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