Diggory is a weirdo. Really. He's a recluse, a loner who rather melodramatically adopted an outcast profession after he got his heart broken. After a break-up, most people just listen to sad music. But Diggory literally marks himself as sad and lonely by quitting his normal, profitable job as a dairy farmer and becoming a reddleman.
A child's first sight of a reddleman was an epoch in his life. That blood-coloured figure was a sublimation of all the horrid dreams which ha afflicted the juvenile spirit since imagination began. (1.9.3)
By becoming the reddleman, Diggory essentially becomes two people. On the one hand, he's the lovelorn Diggory Venn, who pretty much bases all his life decisions on Thomasin Yeobright and his unrequited feelings for the gal. On the other hand, he's this larger-than-life "reddleman" figure of myth and fear.
We can see Thomasin's influence acting on Diggory in a lot of ways. There's his job choice, of course. But we also see Thomasin's influence in Diggory's interaction with Eustacia and Damon:
Rejected suitors take to roaming as naturally as unhived bees; and the business to which he had cynically devoted himself was in many ways congenial to Venn. But his wanderings, by mere stress of old emotions, had frequently taken him an Egdon direction, though he never intruded upon her who attracted him tither. (1.9.9)
Because of his desire to help Thomasin, Diggory adopts the role as the book's primary meddler. For someone who is such a loner, Diggory really gets into everyone else's business. Diggory actually plays a big role in moving the novel's plot along, which contrasts to his status as an outcast. Diggory mirrors the heath itself in this regard, since the heath is very distant and untouchable in a lot of ways, but also interferes with other people (usually to kill them).
Aside from being the book's number one busybody, Diggory is also a man of action. With his red color, he really could be some sort of weird super-hero, who uses his powers of gossip and stalking to make people do things. Or something like that.
We see Diggory's mixture of aloofness and meddling come out most often in relation to Damon Wildeve, interestingly enough. The two are rivals, and we can really see how cool and collected Diggory is in comparison to Damon.
"Five more!" shouted Wildeve, dashing down the money. "And three casts be hanged – one shall decide."The red automaton opposite lapsed into silence, nodded, and followed his example. (3.8.19-20)
Diggory is like a cross between a robot and James Bond in Casino Royal here. At any rate, Diggory is an active, take-charge kind of guy. Not so much with Thomasin, but he does manage to get the girl in the end. However, we do have to wonder how he'll fare in domestic, married life. Diggory really seems to relish his nomadic, wandering lifestyle. And he also has another love which may cause problems with Thomasin.
"Yes, I am given up body and soul to the making of money. Money is all my dream."
"O Diggory, how wicked!" said Thomasin reproachfully, and looking at him in exact balance between taking his words seriously and judging them as said to tease her. (6.2.35-6)
This passage actually shows how suited these two are for each other. Thomasin's reaction to Diggory, a balance between seriousness and humor, mirror Diggory's own attitude towards life, which we can see with his very dry sense of humor. However, Diggory isn't entirely joking here about money, given how hard he works, how frugally he lives, and how skilled he is at gambling.
In the end, Diggory may be more like the other characters in the novel than we'd originally thought. His marriage to Thomasin may partially be the result of the illusions he held about what marriage to Thomasin would actually mean.
So we mentioned earlier that Diggory is like two people. As a reddleman, Diggory becomes a figure of legend and scary campfire stories. To the people of the heath, the reddleman was like the bogeyman or like Jason in his hockey mask – he's a figure of fear.
"The reddleman is coming for you!" had been the formulated threat of Wessex mothers for many generations. (1.9.3)
So Diggory's role as the reddleman links him to themes of history and tradition. He's a huge part of the culture of the heath itself. Which begs the question: what exactly does Diggory become, and what is his new role when he washes the reddle off and looks like a normal guy again?