Inhabitation Ally Spirit PC Options

[Symphonic]

I have what I think is an interesting idea for a character, the problem is I have no idea how to build him and wanted something a little more concrete before going to my GM. A friend directed me here for advice. The idea briefly is this: a suicidal houngan decides to give himself over to the loa (read inhabitation by ally spirit). So the player would in essence be the amnesiac ally spirit inhabiting the houngan's body (preferably in a hybrid merge). Is there any way to do this at chargen (to fit the concept) and without totally unbalancing the game?

The second question is really my main concern. I know this could be done using Karmagen (though again the details I'm not quite clear on - would you build the houngan first, then the ally and combine the two or just build the ally?), but I don't want to ruin the game or my enjoyment with a completely overpowered/underpowered character. Can it be done?