I thought nano-banana would not be the best since I wasn't getting good results.
"Create a very low pixel art character of a vampire hunter facing mostly right. Should be 64x64. The only weapons he should hold are a whip. He should have blue hair and green clothes and a small color pallette.":

Then tried a different prompt:
"Create a 2d 16-bit retro pixel art sprite of a heroic warrior, in a wide combat stance. He has blue hair with a gold headband, a sleeveless green tunic with gold trim and a gold belt, bare shoulders, grey pants, and long dark grey gauntlets and boots. His left fist is clenched and raised; his right hand holds a looped golden-yellow rope.":

Then tried again with a special /skill provided by another graphics software engineer:
"Create a 2d 16-bit retro pixel art sprite of a heroic warrior, in a wide combat stance. He has blue hair with a gold headband, a sleeveless green tunic with gold trim and a gold belt, bare shoulders, grey pants, and long dark grey gauntlets and boots. His left fist is clenched and raised; his right hand holds a looped golden-yellow rope.":

Then asked it to update the second one with a whip instead of a weird lasso. But it just did a bunch of weird stuff and bad edits. Seems like antigravity can't really edit existing images, but can just wholesale regenerate. Unfortunately there isn't a ton of consistency here without inputting the prev image.

"Create a 2d 16-bit retro pixel art sprite of a heroic warrior, in a wide combat stance. He has blue hair with a gold headband, a sleeveless green tunic with gold trim and a gold belt, bare shoulders, grey pants, and long dark grey gauntlets and boots. His left fist is clenched and raised; his right hand holds a looped golden-yellow whip with a distinct black handle.":

It looks a little better at native resolution without the antialiasing.

"vampire hunter looking mostly right and facing towards the camera. Holding a loop of rope and empty offhand. Style is very simple pixel art.":

And I just yolo did it again with a higher resolution.

"vampire hunter looking mostly right and facing towards the camera. Holding a single loop of rope in one hand and nothing in the other hand. ":

Finally at 128 seems to be the best even though on screen it's gonna be tiny.
"16-bit retro pixel art sprite of a heroic warrior, in a wide combat stance. He has blue hair with a gold headband, a sleeveless green tunic with gold trim and a gold belt, bare shoulders, grey pants, and long dark grey gauntlets and boots. His left fist is clenched and raised; his right hand holds a coiled, looped golden-yellow whip. ":

128x128. I think default setting? Probably RD Fast.

"character of a vampire hunter facing mostly right. He should be wielding a whip and empty off hand. Green clothes, big gloves, big boots, and blue hair. ":

64x64
"vampire hunter looking mostly right and facing towards the camera. Holding a whip and empty offhand. Style is very simple pixel art.":

"vampire hunter looking mostly right and facing towards the camera. Holding a loop of rope in one hand and nothing in the other. The empty hand is posing in front like he is going to block.":

I kind of forgot this even existed. So bad I just threw it away.
"vampire hunter face looking right and body facing camera. Holding a thick loop of rope in one hand and nothing in the other.":

"vampire hunter who is looking right. Holding a thick loop of rope in one hand and nothing in the other.":

So maybe it was the prompt not being detailed enough. Gave antigravity a pic of the MC sprite from vampire survivors and asked it to describe it so another LLM can regenerate it. Then condense the intense file long description into a single prompt.
"16-bit retro pixel art sprite of a heroic warrior, black outlines, white background. The character faces right in a wide combat stance. He has blue hair with a gold headband, a sleeveless green tunic with gold trim and a gold belt, bare shoulders, grey pants, and long dark grey gauntlets and boots. His left fist is clenched and raised; his right hand holds a coiled, looped golden-yellow whip. Grounded by a small black pixel shadow under his feet.":

Switched to 3 person character rotation setting.

"16-bit retro pixel art sprite of a heroic warrior, in a wide combat stance. He has blue hair with a gold headband, a sleeveless green tunic with gold trim and a gold belt, bare shoulders, grey pants, and long dark grey gauntlets and boots. His left fist is clenched and raised; his right hand holds a coiled, looped golden-yellow whip.":

So it turns out for retro diffusion the settings do matter quite a bit. And having a detailed prompt.

"16-bit retro pixel art sprite of a heroic warrior, in a wide combat stance. He has blue hair with a gold headband, a sleeveless green tunic with gold trim and a gold belt, bare shoulders, grey pants, and long dark grey gauntlets and boots. His left fist is clenched and raised; his right hand holds a coiled, looped golden-yellow whip. ":