Shroom may have died before our book even begins, but he's still one of the most important characters in the whole thing. His thoughts on life, love, death, and survival have had huge impacts on Billy's impressionable mind, and his death is a great turning point in Billy's life.
Shroom reads books, like, all the time, which is supposedly where he has learned all of the obscure factoids he likes to just spout off as helpful little tidbits. This guy's got a brain, folks, grunt or not.
So, yeah, okay: Shroom is a weird dude. He is also weird-looking, something that just helps to play up his Yoda-like knowledge of arcane belief systems and random trivia:
He was heavy into the whole ethnobotanical mystic trip and even looked like a giant shroom, a fleshy, slope-shouldered, melanin-deficient white man with the basic body type of a manatee, yet he possessed a prodigious blue-collar strength. He could one-hand the SAW like a pistol and ready-up the .50 cal, and forty-pound sacks of HA rice were like beanbags in his grasp. Every other day he shaved his head, a surprisingly delicate orb that seemed a couple of sizes too small for the rest of him. In heat conditions his face lit up in swirling lava-lamp blobs, and he didn't so much perspire as secrete, producing an oily substance that covered his body like a slick of stale pickle juice. (Virtue.51)
We can't help but picture Shroom as this guy for some reason—which, actually, now that we think about it, makes perfect sense because, you know, Krang is just a big brain, and so is Shroom.
Shroom, for example, is able to tell Billy all about the ancient Sumerians, whose culture and belief systems were supposed to have influenced half the stuff in the Bible. He's also able to recite Inuit tales about shamans who can predict the day you're supposed to die just by looking at you. Shroom expands Billy's world. He opens Billy's mind up to question of philosophy, religion, world culture. The world, Billy learns, is bigger than Stovall, Texas. It's even bigger than the United States.
Billy is so convinced of Shroom's godlike, omniscient qualities, in fact, that he thinks Shroom even predicted his own death:
Shroom, Shroom, the Mighty Shroom of Doom who foretold his own death on the battlefield. […] Shroom turning in his seat just as they hit the s***, Mango already opening up on the .50 cal as Shroom reached back and took Billy's hand. "I'm going down," he yelled into the racket, which at the time Billy heard as "It's going down," his ear rounding off the weirdness so the words made sense. (Human Response. 47)
That's crucial, folks. When Shroom and his mystic ways aren't powerful enough to survive the war, Billy is there to hold him as he dies. That changes someone, you know? His friend (and guide to life) is gone, and there's nothing he can do about it. Ever since, Billy has been desperately trying to connect with someone else who knows what it's like to lose someone the way he lost Shroom—but he ultimately fails to find anyone who can help him process Shroom's death.
Death just isn't something people want to think much about in places like Texas Stadium.