What do we learn about Jim in the last chapter of The Adventures of Huckleberry Finn?
Your question implies that readers learn something new about Jim in the last chapter of The Adventures of Huckleberry Finn by Mark Twain; however, we are given no new or surprise information about Jim in chapter 43 of the novel.
What we do discover is that Jim has been keeping a secret from Huck. When he and the two boys are talking about whether the six thousand dollars would still be there or whether Huck's pap has taken it, Jim reveals something quite sobering. He reminds Huck of an incident early in their journey in which there was a houseboat floating near Jackson's island on which Jim discovered a dead man's body. At the time, he kept Huck from seeing it, and now Jim reveals that he did so because the dead man was Huck's pap. Jim says:
"Doan' you 'member de house dat was float'n down de river, en dey wuz a man in dah, kivered up, en I went in en unkivered him and didn' let you come in? Well, den, you kin git yo' money when you wants it, kase dat wuz him."
While it is a sobering revelation for just a moment, this is actually good news for everyone. No pap, plenty of money, and freedom.
The only other possible revelation concerning Jim in this chapter concerns his chest hair. Jim always believed his hairy chest was a predictor of good luck, and perhaps he is right, since he got forty dollars from Tom.
So, what do we learn about Jim? Not much. What do we learn that he did? He kept Pap's death a secret for the entire journey until now, when it matters most to Huck.