In the novel They do it with Mirrors, Arthur Jenkins is a juvenile delinquent who lives at the Institute. He is described as "a fair-haired boy with pink cheeks and a suspiciously angelic expression".
When Ernie Gregg is missing during roll call, Arthur is questioned, as he was the last person to talk to Ernie before he went missing. Arthur says that Ernie had not told him anything, but had been enthusiastic about the play at the theatre. He also says that Ernie had not been out on the night of the murder, and had never gotten out at night at all. He says that Ernie boasts that he can pick locks, but is actually no good with them.