Wally is the brains of the Tunnel Snakes. He doesn't talk as much as the others, but they listen to what he has to say when he does say something. He is one of the Lone Wanderer's peers in Vault 101 and is encountered throughout the Lone Wanderer's youth inside Vault 101.
During their formative years, the Tunnel Snakes harassed other students in Vault 101, even being bold enough to taunt the Overseer's daughter, Amata Almodovar. Butch DeLoria presumes to be the leader of the Tunnel Snakes, but Wally Mack might disagree if challenged.
Growing Up Fast: Wally attends your 10th birthday party and takes part in discussions leading to the formation of the Tunnel Snakes. Wally makes fun of The Lone Wanderer's Pipboy, since he never wears his. Also, he makes fun of the Lone Wanderer's birthday party, saying that the party is for the little kid and that on his birthday, everyone played Hunt the Mutant in the Atrium. He then notes that Lone Wanderer wasn't invited to his party.
"This party's pretty good, I guess. For a little kid. Balloons. Party hats... Not as cool as my party though. Remember how my dad got Andy to do magic tricks? And we all played Hunt the Mutant in the Atrium? Oh, right... you weren't invited to my party. Too bad, man. It was really fun."