Meursault seems to care about no one but himself and nothing but his physical needs. I will argue that Matthew is a direct foil for Meursault.
Matthew loves Marie and fixing radios. He is also passionate in the inverse: he hates his dad and fixing televisions. Yet Matthew is still an existentialist. He makes rash decisions like inviting Marie into his home, and has a blase attitude about death by carrying around a hand grenade “just in case”. As we’ve seen in The Stranger, Meursault holds little value to marriage, expressing to Marie that he would marry any other woman who asked. Matthew, on the other hand, outright proposes to Marie several times, even conceding that he loves her by agreeing that respect, admiration, and trust equal love.
Had Meursault been at the kitchen table with Trust-Marie’s mother, he would have agreed that he probably would’ve gone for Peg since she’s prettier. I’m a bit worried about Meursault having access to a hand grenade though, so let’s journey away from this plot-crossing.
I would say that Meursault’s existentialism is “every man for himself” whereas Matthew’s is based more on living how he wants to live by picking and choosing which parts of society he wants to live by.