Step parent. While not entirely thankless (depending on the kids involved) it’s tremendously underappreciated.
So much expectation that you do things for kids that aren’t yours.
Don’t get me wrong - it can still be rewarding in many ways, and my stepkids and I love each other like blood. We have a fantastic relationship.
But it gets under my skin every time I think about how little their own father has done for them, and I’ve had to pick up the (financial) burden, yet that prick will be the one who gets to walk my stepdaughter down the aisle.