No, we don't mean the actual one (except maybe symbolically). "Christ" is a nightmarish figure Joe conjures up while tripping on morphine. He's conflated with a sixteen-year-old who is let out of jail on the condition that he join the army; his mother is frantically searching for him at the train station the day Joe leaves. In Joe's vision, his job consists of visiting trainloads of young men bound for death at the front. He also stands on top of trains and screams. See Joe's "Character" bio for more on how the figure of Christ changes throughout the course of the novel.
"Lazarus" is the nickname given to the corpse of a German soldier that the English regiment shoots when they see him walking toward them across No Man's Land. He gets the name because on two separate occasions the soldiers try to bury him, and both times a shell blows up his grave and sends him back to the surface.
Lazarus is a biblical figure whom Jesus resurrects from the dead. The idea of Lazarus is an important one in the novel, considering how Joe waffles between being alive and being dead—and considering how he thinks of himself as someone who can "talk for the dead," just like Lazarus (18.21). A statement like that suggests a comparison between the dead German soldier as a kind of "Lazarus" on a down-and-dirty level and Joe as a kind of Lazarus on a more metaphorical level.
The new subaltern shows up one day while Lazarus is rotting away somewhere nearby. He's young and fresh-faced and innocent, and he probably got his position by being aristocratic and related to someone high up. The others try to shield him from the darker realities of war by not letting him go on night patrol, but he sneaks out one night and winds up arm-deep in Lazarus. He loses his mind and is sent away from the front.
Joe considers it ironic that he has a useless body and a functioning mind, while the Limey subaltern has the reverse. The Limey subaltern illustrates another aspect of war, a particularly new one that came about with World War I: the acknowledgment of severe psychological trauma.
The Colonel is an Englishman who's "a great guy to stand on form" (12.21), which basically means that he likes things to be done in a neat and ordered military fashion. He's from a bygone era and doesn't seem to be in keeping with 20th-century warfare. For instance, he orders Corporal Timlon to take a squad into No Man's Land and risk being shot just to bury Lazarus; then, later, he makes Timlon recite an entire funeral service in the midst of No Man's Land—just because orders must be obeyed.
The colonel doesn't seem to have much sense about the dangers of the war. He's just a stickler for military decorum who orders those below him to do the dirty work. In that way, he's a lot like the people Joe criticizes at the end of the novel—those who orchestrate or call for war without having to fight it themselves.
Corporal Timlon is the subaltern who has to bury Lazarus. During the second burial round, he gets shot in the butt and has to leave the front, which ends up being lucky for him, since his regiment is wiped out a few weeks later. How's that for irony?
The day nurse is a consistent fixture in Joe's life for the years that he is in the hospital. He imagines her to be large, middle-aged, and gray-haired. She's skilled at taking care of Joe, but she doesn't understand what Joe wants when he starts to tap out messages.
The best word to describe the doctor is "heavy": he has heavy footsteps, and when he taps his finger on Joe's forehead, it's described as a shattering feeling. He likes to shoot Joe up with morphine, and even when he figures out that Joe is tapping out Morse code, he really doesn't seem to care what Joe has to say. He represents authority, particularly of the military variety.
The new nurse arrives one day and traces out "MERRY CHRISTMAS" on Joe's chest. This is the first time since Joe's injury that someone has tried to communicate with him, and he tries to make her understand his method of tapping. She tries a lot of things before eventually figuring it out. Joe imagines her to be a younger woman, based on the softness of her hands.