My No-Name Wife Is the Dragon Queen!

A man dies and wakes up in the body of a python. Lying beside him is a small green snake, and he works out fairly quickly that she is the empress of the Dragon Palace and commands a hundred thousand troops of the water realm. He assumes she will kill him. Then he binds to a system that returns a hundredfold on anything he gives sincerely, so he starts giving. He brings her food. He helps her cultivate. He puts himself between her clans and whatever is coming for them. Every honest act comes back to him multiplied, so the python grows longer and stronger by the day while she recovers her own power. They protect the underwater clans together, and something develops between them that neither is supposed to feel across their positions. She is a ruler with an army and he is a snake who arrived from somewhere else with a hidden advantage, and neither of them has told the other the whole truth about what they are getting out of the arrangement.