Marcus is one of Diana Wynne Jones' most effective child characters. He's willful, loving, cranky, and engaging, and very realistic for a child. Though his age isn't given, he appears to be somewhere around two years old -- he can walk, talk, and occasionally revert to baby tantrums, but he has strong opinions about things, and has a small child's capacity to tell good people from bad people just by the way they feel to him.
One of the key features about Marcus is that he has his own kind of baby speech, and few other people can understand or interpret it. One of the first things that forms a bond between Marcus' mother Zillah and the cadet Tod who comes to rescue the travelers inside the mystery capsule is that as soon as he begins speaking to the stranger, it's clear that Tod understands exactly what he's saying. The same thing is true of Herrel Listanian, whom Marcus meets in a setting that isn't safe at all. Still, he understands everything Marcus says, which is reassuring to both Marcus and Zillah under the circumstances.
The High Head of Arth, however, is another matter.
"Little boy, what is your name?" Marcus looked up under his mother's chin, a stormy blue glare, and gave another body-shaking sob. "Barker."
Odd name. "And where do you live?" asked the High Head. "Idanda how" said Marcus. "Dilly bool." He turned his face away.
"And how did you travel here?" persisted the High Head, with a strong sense of getting nowhere. "Bud," said Marcus. "Jidey bud. Didden lie bub. Go bub. Doe lie did how. Wan hoe, wan hoe, wan hoe, Dillah! Wan gorblay, wan bregia, wan barberday, wan doad!" By this time he was bawling disconsolately. "Dillah, I need DOAD!"
"What is wrong with him?" the High Head said, giving in again. "Why is he crying?"
And Zillah has to explain that Marcus is hungry, not having liked the food he was given, and that he is yearning for bread, and marmalade, and toast. She doesn't add that Marcus didn't like the shiny bus they came in, or that he doesn't like the house they are in now, or that Marcus considers the High Head a silly fool. There are some things you don't say to hostile magicians wearing hats with long curling horns.