I haven't written as much about Elrohir and Belemir, but they aren't 'also-rans' to me. Elrohir, despite his quick temper, is a well-regarded diplomat and government leader, as well as a military leader. He was Elrond's ambassador to Arnor's government in Annuminas for decades at a time. Elrond trusts Elrohir to govern Imladris in his absence more than he trusts Elladan, because Elrohir is less easily distracted. Between the two twins, Elladan is the genius, but Elrohir is the one with both wisdom and steadiness. He's also a military genius.
As a warrior and military leader, Elrohir is one of the best strategists and tacticians whom Glorfindel has ever trained. In part, that is why Elrohir didn't make as much time as Elladan did for learning different disciplines. Elrohir specialized as a warrior and officer even more than Elladan did as a healer. Elrohir is a military historian, and Glorfindel's alter-ego in the field, when Glorfindel isn't there himself to make decisions for keeping Elrond and his children safe and Imladris protected. In the army of any other kingdom on Middle Earth, Elrohir would be a senior captain who sometimes served as a general. Imladris' permanent standing guard is a relatively small and flexible organization, although every elf in Imladris is trained to defend themselves and others. Glorfindel's officers all take the rank of lieutenant rather than anything higher, because Glorfindel never desired to be promoted above Captain (for several different political reasons, some of them Glorfindel’s and some of them Elrond’s).
Elrohir is actually a permanent lieutenant in Imladris' guard, whereas Elladan is not. Elladan always goes with his twin brother whenever Elrohir leaves Imladris to take on a military task, but in Imladris, Elladan often goes to reserve status and is only a militia officer, which has permitted him time to pursue his own interests as a healer. Almost every skill that Elladan has ever mastered is connected either to his profession as a healer, or to his dedication to the warrior-arts. The reason for that latter dedication is his love of his twin, and his desire to guard Elrohir's back, and be able to help Elrohir with the responsibilities of military leadership. If it weren't for Elrohir, and Elladan's being Elrond's second-heir, Elladan would never have become more than competent to defend himself and others. He's not a warrior in his heart, although he's managed to mostly hide that from Elrohir, so that Elrohir has someone to rely on.
As a side note, Elrond also is not a warrior in his heart, although very early training by Maedhros inclined him to the warrior arts with dedication serious enough to nearly match his military- minded twin. Further training from Glorfindel after his arrival on Middle Earth later honed Elrond’s skill at arms to a nearly peerless level.
Like Elladan, Elrohir is a master weaponsmith and blacksmith. He's also a master tanner, since armor is sometimes made of leather as well as metal. He's also captained military vessels along the Lindon coast and in the large lake by Annuminas (Lake Evendim, I think?).
Elladan has fallen in love with three human women during his life, and almost married one of them. Elrohir always followed his brother whenever Elladan spent time with the women he was in love with, but he always secretly hoped that the romances wouldn't work out (which they didn't, in part because Elladan couldn't bear to be parted from his twin for most of eternity).
In the West, Elrohir will marry Ereinion Gil-galad's oldest daughter and heir Rissaurel, and will sometimes be Rissaurel's King and co-ruler when she rules Ereinion's kingdom in the West. Elladan takes a long time to marry in the West, because he never loves an elleth with the same passion that he'd had for the human women he'd fallen in love with in Middle Earth. Eventually, he and Rissaurel's next oldest sister, Ciryawende the scholar and sailor, become close friends and decide to marry. Love comes for them in time, but Ciryawende, like Elladan, probably would have fallen in love with a human if she'd been born on Middle Earth (a human male, incidentally, as most of Ciryawende’s few romantic liaisons before Elladan were with elven males).
Elrohir is a fully trained healer, although only since about fifty years prior to the Ring War. Before that he was a highly competent battle field healer but not fully trained. His twin and his Uncle Orophin being badly injured in Mordor during their ninth mission for Mithrandir (although no one knew what they were actually doing there) is what convinced Elrohir that he needed to become a fully trained healer.
Elrohir is also a poet and a well-respected author of philosophy treatises and fiction books as well as military histories. He keeps the former completely quiet, even Elladan doesn't know about all of it. Only Melpomaen does, because Melpomaen is Elrohir's editor and publisher (through publishing companies established and 'run' by Elladan, although actually mostly run by Melpomaen).
Under a pen name, Elrohir also writes children's books (the 'Little Elfling and' series, which was Legolas' favorite and then Aragorn's favorite when they were growing up). Elrohir first started writing the books as a gift for elfling Legolas. The series began with 'Little Elfling and the Scary Wizard,' and more titles followed. Then Elrohir wrote some new stories for the series when Aragorn was growing up as his foster-brother Estel, including 'Little Elfling and the Bad Kitty,' which chronicled the time that Estel met a wounded mountain lion.
Elrohir is the 'leader' amongst all of his siblings, and in the threesome of himself, Elladan, and Melpomaen. The three of them, and in particular the twins, complement one another. Elladan has the widest breadth of knowledge, Melpomaen usually supplies the legal, economic, and scholarly side, and Elrohir uses them both as resources in deciding how to use what they know and what they can find out to achieve any given objective. That's not to say that Elladan and Melpomaen never play the role of the leader, but normally it is Elrohir. He's also the one whom his siblings come to first when they need to get things done. Elladan is the one they usually come to for a sympathetic ear, but it's usually Elrohir they find if they need other help, or someone to help them fix a mistake they've made. Again, the twins complement each other in taking care of their younger siblings.
Belemir is a less thoroughly explored character, in part because of how early he died. He was gentle but brave and capable, like Faramir. He was his slightly younger sister Arwen's best friend. He was a great warrior, and not just skilled but also very cunning. But, like Faramir, Elrond and Elladan, Belemir wasn't a warrior in his heart. He became a warrior and a war-leader because of who he was - Elrond's son - and because Middle Earth and Annuminas needed a warrior and a war-leader.
He was only the equivalent of two years older than Arwen, and she was his best friend. Of all of Elrond's and Celebrian's children, they are the two who felt the greatest affinity for humans. Arwen was a lady-in-waiting to the queens and princesses of Arnor in Annuminas for much of her early life, and Belemir served as a knight of Arnor and a captain in Arnor's army. Most often, he was an armed companion and advisor to the kings and princes of Arnor.
Belemir was a scholar and a musician, and he had that quiet part of his nature in common with Melpomaen and even Elrohir. I expect that Belemir co-wrote treatises and books with Elrohir at times. Belemir had a quieter sense of humor than the twins, but he was also in some ways a more inspiring leader than either of his older brothers. He had a knack of remembering everything about everyone he had ever met, and knowing what to say to inspire them. Again, he was a lot like Faramir.
Even more than any of Elrond's other children, Belemir and Arwen always did what they thought was right, no matter the consequences, and they could always count on one another for support.
Belemir married Princess Rossidhiel, the older sister of Arvedui, the last King of Arthedain (the longest surviving successor kingdom to Arnor). She was, of course, human, so he made Luthien's choice to accept a mortal life span and go to the human Halls of Mandos (rather than immortality with the possibility of being reborn in the West like an elf).
Arwen was deeply saddened by losing her 'twin' to mortality, but at the same time she wanted what was best for Belemir, and was happy for him. She was one of their wedding attendants, and stayed in Annuminas as one of Rossidhiel's ladies-in-waiting.
When Arwen was on a visit back to Imladris, the Witch-King of Angmar attacked Annuminas. Belemir helped his brother-by-law King Arvedui to direct the defense of Annuminas during the Witch-King's siege. Belemir went to meet the Witch-King in single combat, knowing that he almost certainly could not win, in order to cause a distraction so that messengers could escape Annuminas to take word of its plight to Imladris, Gondor, Lindon, Mithlond, and their human allies the Lossoth who lived to the north of Mithlond.
Belemir succeeded in his aim, but he was killed by the Witch-King. Rossidhiel was pregnant with twins when her husband Belemir died, and then she died not long after (and before their children were born). She drowned with her brother Arvedui before they could be rescued.
I see Belemir most clearly through Arwen's eyes. He appears with Arwen in a few of the Tales of Imladris ('Yuck' and 'Stolen Child', I think) as well as being the inspiration for Arwen's fighting on in "Dedication" in Tales of the Third Age in Twilight.
Faramir reminds Arwen strongly of Belemir, as I wrote in "Dedication." Faramir's daughter, Mithiriel, reminds Arwen of both Belemir and Rossidhiel. There are more reasons for these resemblances, as is explained in “Changeling Child,” in the “Lucky Fall” AU (wherein Boromir lives, but everything else is pretty much the same), and in my unfinished story 'The Valiant Die But Once.’