Ben Hargreeves (00.06 The Horror) possesses monsters under his skin from other dimensions. He is deceased, but no information regarding his death is given. There is a memorial statue of him located in the front of the Academy. Even though he's been dead since before the start of the series (but after the Murder Magician affair), he's been portrayed as a member of The Umbrella Academy since ever, appearing as his statue counterpart, or as images of him alive.
In The Umbrella Academy: Apocalypse Suite, The Rumor stands beneath his statue, talking to Spaceboy of how everyone blames him for what happened to Ben, although no one knows for sure what happened the day he presumably died.
His statue was destroyed by the White Violin near the end of the Apocalypse Suite story.