He doesn't need a harem to make Miru jealous.I can and I am sure Ocean does as well, but the game did not reach the point where a scene like this would make sense story wise, William only got home 3 days ago, even the most chad of AVNs MC could not build a harem in this time frame. I am sure that we will have scene where this aspects are gonna be explored but is just way to early for that, let the man cook.
They already talked about how she 'threw a chair at his female friend for being too close to him'. Just make that scene a reality => then make frustrated Miru suplex William into the bed => then make him tease her about "It's normal. You'll do the same with a male friend." => at which she'll go ballistic - "Fuck no!" - and she will make it the angriest fuck of their life.
That will show both her jealousy and - the last part - her extreme definition of loyalty.
Profit.
//It's just an example, of course. There could be trillions of variations.
//But you don't need a harem to show Miru's character.