Marmite - holy moly, who was the guy inventing that rubbish? I tested it once when I was on visit in England and I find it horrifyingly bad, like biting into a yeast block.
If you want something looking similar to Marmite, but tasting vastly better and similar vitamins, try German "Goldsaft" or "Fenner Harz", which is made from sugar beets and comes in lots of variants, e.g. with apples, plums or cinnamon.
But back to the main topic. I like Heather, because she is a good friend to the FMC (and one for the MMC too in the sharing path), if even she has some ideas... She is a wild child and at least in the sharing path she likes the MMC as well. And would not be adverse to get a piece of him herself. Looking forward to see what her story is and how she fits into our main pairing. There are several paths possible, with and without pregnancy.(Speculation: might even come as far as the FMC pregnant by another guy, while the MMC puts Heather in the club for the more extreme versions of the sharing path)
As for the cheat/cuck paths here Heather can be a whirlwind, disturbing our main pair with her wild lifestyle. And here her view of the MMC is different. The Heather on the sharing path is wild, but not vindictive. The Heather on the other paths will gleefully show the MMC nd others their failings quite in the face.
But whatever the case, while there are some "stock" characteristics to Heather, I am sure we will see more of her original story and traits in coming parts of WMW.