A very non-exhaustive list of ideas:
1. Good intentions. The most sympathetic are often not the highest-minded. Being a nice guy who just wants a fairly pleasant life is about the strongest thing I can think of. Moral crusaders are often a bit weird and inflexible (Gandhi, Lancelot, Churchill) or, once you get past the myth, have done some very dirty deeds (Cromwell, Paul Atreides). On the other hand, you have Jim, who has a wife and young child, and just wants to earn an honest bob working on the space dock - but one day discovers what's being brought in and out in those mysterious containers, and becomes bound by his conscience to intervene. Such a character is easy to identify with, and does what we'd all hope to do in similar circumstances.
2. Moral crusaders. See above, but also note the crusader who has a less-pure element to him. Does the fact that Martin Luther King had an eye for the ladies make him more human, or less legendary, or both? The narrator of Rogue Male is an interesting variant on this: he's articulate, decent and pleasant to meet, but a relentless, obsessive hard case at the same time.
3. The awakening conscience. A character who has previously been a believer in the True Faith, or at least willing to go along with it, starts to have doubts. This requires careful depiction, to avoid too much angst, but it can be very powerful. The character Guthwulf in Tad Williams' Memory, Sorrow and Thorn springs to mind, but there are loads. A variant can be seen in the villain who is either less evil or less stupid than his colleagues for self-serving reasons (being nice to the peasants so they won't revolt). General Rommel would probably fit neatly here.
4. People who are entertaining. Abercrombie is quite good at this, but Peake's Doctor Prunesquallor is a great example, or even the Fool from King Lear. Whenever he appears, he entertains. I've heard people say that good guys are boring, which means that they're writing them badly, or that their idea of a good guy is too limited. There are too many angsty tough guys in films and computer games, punctuated with the occasional (highly annoying) "wacky joker" type. A character who is genuinely likeable to be around, without being arrogant, will probably be likeable to read about.
5. People who are simply interesting. To my mind, Frank Herbert made a great mistake in killing off most of the more bizarre characters in Dune and replacing them with a chorus line of stillsuited jihadis.
All of these are people you can empathise with, in the sense of enjoying reading about them and not wanting them to die. I think 1-3 are more obviously sympathetic. To my mind, it is important, whilst being aware of a character's role or function, to treat them as an individual. If "nice guy with family" seems boring "nice guy with dying mother and wife who is a pioneering biochemist" is more interesting. Perhaps the easiest way to create sympathy, though, is to throw a lot of rubbish at the character and see them struggle to survive. Too much comfort can make characters seem bland or conceited.
I regularly write about four characters, two of whom are high-minded idealists, one of whom is a regular joe who just wants to stay alive (but has a concience) and the fourth is a bloodthirsty warrior. But they're all pretty decent, they remain broadly honourable and hopefully have a combination of good intentions and interesting quirks to stay sympathetic, especially when they fight against bad things.