Christian might be the most pathetic person in the entire novel. He's like Charlie Brown – he's perpetually sad-sack and sort of descends on a crowd like a wet blanket.
"I'm the man"
"The man no woman will marry."[...]
"Yes, I be he, and it makes me afeard," said Christian. "D'ye think 'twill hurt me? I shall always say I don't care, and swear to it, though I do care all the while."
"Well, be dammed if this isn't the queerest start ever I know'd," said Mr. Fairway. (1.3.75-80)
Yikes. But what purpose is the perpetually-depressed and depressing Christian playing here? Well, he definitely shows us how families are far from cohesive and similar. He's pretty much the polar opposite of Grandfer Cantle (we're not even sure how these two can possibly be related).
Christian is also notably weak-willed and his bad decision-making skills contribute to the entire gambling episode fiasco, which temporarily cost Clym his inheritance and worsened his already-weak relationship with his mother. We don't really see much of Christian after that episode, probably because he was shunned by Mrs. Yeobright.
In his pathetic nature, Christian also provides an element of comic relief in the book. His tragedies are so over-the-top and even absurd that they become funny, in contrast to the tragedies that impact the major characters of the novel. But Christian also is a tragic figure and he gives us a glimpse into a different kind of loneliness and sadness. The main characters aren't the entire world of the heath, and characters like Christian help to remind us of that.