Use your best judgment.
If you had inspired your character from a well known character from another game, would you have been guilty of violating copyright? No? Then why would you even consider tagging your game as using AI material?
Because that is the root of the issue. Authorship. The legal situation about authorship from AI material is not established. There might be changes and ambiguity. If you create an asset and another game uses it, there might be consequences later on. Therefore it is crucial for any asset to be tagged properly in that regard.
For games the tagging unfortunately is implemented quite bad. There is not really an easy to use filter for AI and no AI. Or rather there is, but it does not heed nuances. It does not even distinguish between having fully made AI pictures and small portions of suggested and modified code. The first will be the thing that players will want to filter out. The second is something that only purist AI avoiders will want to filter out. And your situation is what the extremest of AI avoiders will want to avoid.
So do AI avoiders think like vegans? Some do. But only some.
Let's reverse the situation and it might get clearer. Someone is actually gonna look for AI games. https://itch.io/games/tag-ai-generated
Will that someone be satisfied to find your game in the AI generated set? If you can answer that with a confident yes, than use the AI disclosure.
The situation on Steam is more nuanced. There are no filters for AI that I know of. But the disclosure on the games is very nuanced. I guess you could write in text what you wrote in your op. That AI was used to brainstorm character concepts that were fleshed out by humans.