"Sometimes I worry about you, Bevvie. Sometimes I worry a lot!"
Alvin "Al" Marsh is the spouse of the Elfrida Marsh and the insane, abusive father of Beverly Marsh in the IT (book). Like Butch Bowers and Eddie Corcoran's stepfather, Richard Macklin, Al abuses his daughter. Unlike Butch and Richard, he tends to beat Beverly for odd reasons. While beating her, he constantly tends to say "Sometimes, I worry about you, Bevvie, sometimes I worry a lot."
He abuses Beverly and her mother and acts misogynistic. He hates it when she hangs around with boys. He is last mentioned during Beverly's childhood as trying to kill her out of anger to Its influence. He is also a form that It uses to frighten Beverly since she is afraid of him. In the novel and the miniseries, he dies of unknown causes, possibly he was eaten by It.
He died from unknown causes in 1980. He also appeared as one of Its manifestations, taunting Beverly into getting out of Derry.
Here Alvin is the widowed spouse of the late Elfrida Marsh.
It Chapter One & Two
Here, his wife Elfrida has already been dead for an unknown amount of time, and seems to have past from cancer. IT: Chapter Two (film) reveals he celebrated her birthday by spraying he perfume, even on Beverly, and holding her clothing. He laments that Beverly looks like her, but would be ashamed of her actions by hanging around with boys. In IT (film) Beverly manages to seriously injure him while he attempts to rape her (however you can hear him groaning later on hinting he is alive). It is also heavily implied that he was sexually attracted to and molested his own daughter.
Cary Fukanawa's Alvin
In the 2015 to 2016 scripts of the film, Al's character hits most of the beats of he does in his novel version. At least, he comes off that way. In his mind, anyone who lives under his roof is his property. And so, he treats Beverly with that mentality. This later leads to Alvin attempting to rape Bev, resulting in what would later be formed into the scene where Beverly beats him over the head with a toilet tank lid.