Abishai, the brother of Joab, is one of David's most fearsome warriors. He's not quite one of the three greatest soldiers in the whole army ("The Mighty Three"), but he's still an exceptional one and performs feats of military prowess, like killing a Philistine giant named Ishbibenob. So, like Joab, he's rather a violent dude. Also, he commands a third of David's army when it fights against Absalom.
David sends Hushai the Archite behind enemy lines to pretend to be a traitor and work in Absalom's court. Hushai gets Absalom to overcome his initial suspicion, and he gets him to accept his advice over that of the wise counselor (Ahithophel). Hushai's advice leads Absalom into the battle against David's army, which proves to be his doom.
Shimei is a former member of Saul's house. When David retreats from Jerusalem, after Absalom invades, Shimei curses and throws stones at him. But, when the war ends, Shimei begs David for forgiveness. David is compassionate towards him, demonstrating his noble and generous side. So Shimei, in the story at least, functions as an opportunity for David to be a good guy, demonstrating the qualities that keep the reader on his side.
Amasa is the leader of Absalom's army and a very skilled general. However, after Joab kills Absalom, David replaces Joab as head of the army with Amasa—which, given that Amasa had fought on the opposite side, was like a slap in the face to Joab. So, to get his revenge, Joab kills Amasa, slicing his guts open with one slash of his sword.
Sheba is a rebel who manages to lead a popular uprising against David. He comes to a bad end, however, when he gets trapped in a city, besieged by Joab and David's army from the outside. The people in the city aren't thrilled at the situation, so after a fruitful discussion between one of their wise women and Joab, they decide to cut Sheba's head off and throw it over the walls to the Israelite army. This decision leaves everyone happy… uh, except for Sheba, obviously.
These guys are priests who stay behind in Jerusalem when Absalom takes over. They act as spies for David, sending info back to him through their sons. The more you know…
King Hanun of Ammon suffers from a lack of trust. He believes that David's emissaries to his court were spies, trying to help David collect intel for an invasion (and, hey, who's to say that they weren't?), so he humiliates them, stripping them naked and shaving their beards. This all backfires terribly when David really does decide to attack, winning a decisive victory over Hanun and his ally, King Hadadezar.
Ziba is the servant of Saul's grandson (and Jonathan's son), Mephibosheth. David really likes Mephibosheth, even letting him eat at his own table. However, when Absalom rebels, Ziba escapes, claiming that Mephibosheth turned traitor, and David grants him all of Saul's remaining lands.
When the war is over, Mephibosheth claims Ziba pulled a fast one on him and that none of what he said was true—Ziba abandoned the physically disabled Mephibosheth and told his made-up story to curry favor with David. But Ziba winds up with Mephibosheth's lands anyway. It's possible Ziba was telling the truth—but the story seems to imply that he wasn't, and that Mephibosheth remained loyal.
This guy gets a really raw deal. Basically, he helps Saul, who's already half-killed himself, to finish the job—at Saul's request. In return for killing "the Lord's anointed," David has this helpless Amalekite killed. Unfortunately, Amalekites are usually the ancient equivalent of "cannon fodder" in these books—and the victims of a full-scale genocide conducted by Saul (at God's behest) in 1 Samuel (though somehow, after supposedly being totally annihilated, they keep reappearing).
These guys race to give David the news about the outcome of the battle with Absalom. Ahimaaz—perhaps, respecting David's feelings—doesn't want to tell him that Absalom was killed (though this is the piece of news David most wants to hear). Ahimaaz gets there first, giving him this incomplete news about their victory—but the Cushite arrives slightly later, bringing the terrible news of Absalom's demise.
King Solomon—so important in 1 King—is just a baby in this story, and his birth is simply noted, before the story moves on. The other wives and concubines and children (aside from the big players like Bathsheba and Absalom) are also simply noted in passing. The only time when they play a more substantial role comes when Absalom sleeps with his father's concubines—the "point of no return" for him and his rebellion.
Earlier, we said that Uriah gets a pretty raw deal. Well, so do these guys. Simply because their dad persecuted a people called the Gibeonites, the sons of Saul are offered to the Gibeonites in exchange for God ending a famine he has inflicted on Israel. The Gibeonites kill the sons by impaling them, right before the barley harvest happens (which is apparently successful, because the famine ends.) It's a particularly horrible and graphic instance of sons being punished for the sins of their father.
This is yet another example of a guy who becomes a victim of the various conflicts that plague 2 Samuel. Paltiel, the husband of Michal, loses her when David wants to remarry her in exchange for allowing Abner to join his side. Paltiel trails behind Abner, weeping for his wife to be returned, but Abner simply tells him "Go home!" Thus, Paltiel is another one of the little guys who lose out in a big way during this competition between heavy-hitters like David, Abner, and Ishbaal. He's a figure of sympathy and pity.
Barzillai is an eighty-year-old man who helps David out during Absalom's rebellion. David tries to repay him with kindness, offering to bring him to Jerusalem to live and eat with him in his palace. But Barzillai, being old, wants to go back to his home and die in the town where he was born.
So instead, David takes Barzillai's servant Chimham with him and promises to do anything Barzillai wants him to do. It's a pleasant anecdote—another example of the basic niceness of David. Also, since his own son has just died, David seems much more likely to act compassionate towards others. The experience has filled him with sympathy for the sufferings of people, not with coldness or bitterness.
The brother of Joab and Abishai, Asahel is another war casualty. Abner kills Asahel when Asahel pursues him during the war against Ishbaal (because Asahel was a fast runner—"light of foot as a wild roe," to quote 2 Sam 2:18). This sparks Joab to later take revenge against Abner, killing him for what he did to Asahel.
Araunah owns the threshing floor where David sees the angel of destruction who has been afflicting Israel with pestilence. David builds an altar, with Araunah's permission, and Araunah sells him the oxen he needs to sacrifice to God and end the plague.
These guys are ruffians and scoundrels—natural born killers in fact. They stage a gristly home invasion against Ishbaal, cutting off his head and bringing it to David. David is not impressed by people who wantonly kill the son of the Lord's anointed, and he has them killed in return. Like David says, it's a little like the part earlier in the story, where an Amalekite helps Saul commit suicide, much to David's displeasure.
These guys are David's crack troops—his "Magnificent Seven" or "Seven Samurai" (except there's three of them, not seven). "The Mighty Three" includes Josheb Basshebeth, who kills eight hundred men at one time, Eleazar (son of Dodo) and Shamah. They all battle fiercely against the Philistines.
One time, they even go behind the enemy's line of defense to retrieve a drink for David after he casually expresses a wish to have one (they're life-time members of the David Fan Club, it seems—the "Beliebers" of their time, as it were). This makes David a little embarrassed, since they did so much for him, and he offers the drink to God instead. The story also mentions another hardcore warrior—Benaiah, the head of David's bodyguard—who does courageous things like killing a lion and a top Egyptian warrior.
King Hadadezar is yet another luckless Gentile king who can't withstand the onslaught of David's army. The ruler of the Arameans, he gets ambushed while building a monument to himself—uh, embarrassing—and loses a ton of men and supplies and chariots. A later attempt to ally himself with King Hanun doesn't work out well either, with David defeating them handily.
These dudes aren't very important, but we mention them nevertheless, for the sake of being somewhat comprehensive. King Toi dispatches his son Joram to be tribute to David, since they're quite glad he's defeated their old enemy King Hadadezer. They shower Dave with silver and gold, which he then dedicates to God (as he did with all the wealth he managed to plunder).
This luckless guy reaches out his hand to touch the Ark when it is brought into Jerusalem—and promptly gets the bug-zapper treatment, dropping dead on the spot. David gets a little freaked out by this—but later, successfully brings the Ark into Jerusalem… And that's all there is to say about Uzzah.
After Uzzah gets killed whilst touching the Ark of the Covenant, David gets nervous and leaves it at the home of Obed-edom the Gittite for a few months. But when David sees the blessings that come raining down on Obed-edom, he decides to go the whole nine yards, and bring the Ark into Jerusalem itself.
These guys exist mainly for the purpose of getting slain by heroic Israelite warriors: Abishai kills the giant Ishbi-benob, who threatens the life of David, for example. Supposedly, these giants are all descended from a group of giants who lived in the town of Gath: there's Goliath the Gittite, who gets slaughtered by an Israelite named Elhanan, and a giant with twelve fingers and twelve toes, killed by Jonathan, David's nephew.