So I figured out how to manipulate the characters in your save file.
I recommend to make a backup copy or create a new save file slot to experiment with.
For your Save 6 slot, you would look in "Mad Island\Mad Island_Data\StreamingAssets\XML" for the "SaveData06.xml".
Create a copy of the .xml file and save it as .txt to edit the save file and keep the code structure.
Best way to find a character you want to edit is to Ctrl+F their name.
If I want to find Reika for example I would Ctrl+F "レイカ" (I'm playing with Japanese language on) and then it should display something like this:
<friendID>31</friendID>
<npcID>5</npcID>
<groupID>0</groupID>
<employ>1</employ>
<charaName>レイカ</charaName>
<nameVisible>false</nameVisible>
<person>
The friendID is ID of the character that's displayed in the top right when you select a character's status. It's also easy to search for a character this way. npcID is the ID of the type of character they are. 15 is for female native, so if I change that value to 15 then Reika turns into a female native. Unfortunately her appearance also changes according to what values are set for the character's appearance, but if you change it back to 5, then Reika will return to her normal form. However, make sure that the character is not wearing or holding anything. If you change Reika to a female native then it will delete her clothes since that character is not compatible with Reika's items. You can use this to change a character that currently can't give birth yet to a normal female native, get them pregnant and have birth and then change them back, so that the child will say that their parent was that character. Of course you can manually just change who the game says the parent is, but that's not as fun.
If you go down a littler further you can manually change the pregnancy of a character. The first value is -1 if the character isn't pregnant and any other value (friendID) will make them pregnant by that character. Takumi is -7 for those that want to have Takumi be the father for any reason. The second value is 0 when they are ready to give birth, and non-zero when they are not. It starts at 12 when they first become pregnant, so I assume the value goes down over time until it reaches 0.
<pregnant>
<int>-1</int>
<int>0</int>
</pregnant>
Further down you can change the relationship stats that they have with other characters. It's pretty self-explanatory, the friendID is their relationship with the character that has that friendID, love is their Love value, and the "sexcount" values correspond to the values from left to right in game between those two characters.
<Lovers>
<friendID>32</friendID>
<love>150</love>
<sexCount>
<int>0</int>
<int>0</int>
<int>0</int>
<int>0</int>
<int>0</int>
<int>0</int>
<int>0</int>
<int>0</int>
<int>0</int>
<int>0</int>
</sexCount>
</Lovers>
There's a lot more, but this about covers the most basic stuff that people probably care about editing for a character.
The entire character's save starts with
<CharaSave>
and ends with
</CharaSave>
so anything between that is for that specific character.