Seb

He is essentially the same character from Thing about Jack (but Toby is not Toby (later Adam) from Jack. I just have a strangely limited number of names for my character)

The reason why I want him in this story is essentially two fold:

Firstly he is a party animal. He is the one who will get them into places where they wouldn't otherwise go. He can score them weed. And he genuinely seems to enjoy his hedonistic lifestyle.

Second he's a genuine confidant. Lois is just wrong. Too intense. Too interfering. And Toby is the opposite, he just seems so uninterested in so much of it Mark doesn't want to go there.

But perhaps Seb is important because he highlights what's good about Toby and Lois as individuals and as a couple. The point about Seb is he is fiercely loyal to both of them. Toby is a friend from school and he knows what a difference Lois has made to his life. And he sees why Toby is so good for Lois.

So if they do split up, Seb is going to be very determined to get them back together and will tell Mark that he was the reason they split and he's going to have to be part of the solution that puts them back together.