At first, Mr. Deas seems to take the side of order over truth, like when he tells Atticus that he has "everything to lose" from defending Tom (15.20). Later, though, he ends up being one of the few people who are willing to take action on Tom's behalf, speaking out in his favor at the trial.
Mr. Link Deas rose from the audience and announced: 'I just want the whole lot of you to know one thing right now. That boy's worked for me eight years an' I ain't had a speck o'trouble outa him. Not a speck."
"Shut your mouth, sir!" Judge Taylor was wide awake and roaring. […] "Link Deas," he yelled, "if you have anything you want to say you can say it under oath and at the proper time, but until then you get out of this room, you hear me? […] I'll be damned if I'll listen to this case again!" (19.89-90)
It's great that Mr. Deas wants to support Tom. The problem is, in speaking out of turn he risks causing a mistrial and drawing out the process even further. But since even with a trial that follows the rules of order, Tom is unjustly convicted, maybe Mr. Deas has the right idea in breaking the rules to speak on the side of truth.
After Tom is killed, Mr. Deas helps support Tom's family. He creates a job for Helen even though he doesn't really need her help. While he doesn't do much to change the ways in which the system is broken, he does make a significant difference in the life of Helen and her children by protecting her from Ewell.
"As Mr. Link came out of his store he saw Mr. Ewell leaning on the fence. Mr. Ewell said, "Don't you look at me, Link Deas, like I was dirt. I ain't jumped your-"
"First thing you can do, Ewell, is get your stinkin' carcass off my property. You're leanin' on it an' I can't afford fresh paint for it. Second thing you can do is stay away from my cook or I'll have you up for assault-"
"I ain't touched her, Link Deas, and ain't about to go with no n*****!"
"You don't have to touch her, all you have to do is make her afraid, an' if assault ain't enough to keep you locked up awhile, I'll get you in on the Ladies' Law, so get outa my sight! If you don't think I mean it, just bother that girl again!"
Mr. Ewell evidently thought he meant it, for Helen reported no further trouble. (27.8-12)
Ewell tries to hide behind the letter of the law, but Mr. Deas sees what he's really trying to do: terrorize Helen in a way that might make her stay home or leave Maycomb altogether. It raises the question: what if Helen herself had taken the matter up with the law? Would she have been able to get support without a white man backing her?
Like Tom, it seems that Helen's legal rights depend not on what the law says but on the white men who enforce—or fail to enforce—the laws. What would have happened if he Mr. Deas hadn't been willing to take on Ewell?