Originally posted by: .Vrish.
Ashwatthama
Vile as he was after his father's death, what's ignored is that while most of the Kaurava warriors were killed either by Arjun or Bhima, most of the Pandava warriors were killed either by Drona or Ashwatthama: very few were killed by Bheeshma, Karna or Shalya, all of who preceded Ashwatthama to the senapati role. Not just that, Ashwatthama also had 2 major weapons - the Narayanastra and the Brahmashira - both of which he misused. He also not only killed Ghatotkacha's son Anjanparva, but also defeated Ghatotkacha when the latter tried avenging Anjanparva.
Satyaki
While his counterparts on the Kaurava side like Kritavarma and Bhurishrava were not among the top warriors, Satyaki was #2 among the Pandavas. If a major warrior on the Kaurava side was causing trouble and Arjun and Bhima were preoccupied, usually it was Satyaki who salvaged the situation
Sveta
This second son of Virata was killed on day 1, but not before making that day his day to shine. Once his brother Uttar was slain by Shalya, he retaliated, forcing Shalya to flee, and dispersed the Kaurava army. That forced Bheeshma to confront him, and even Bheeshma was getting repelled until a heavenly voice told him that Sveta was destined to die at his hands. Bheeshma then repelled all the Pandava warriors who went to support Sveta, including Arjun, Satyaki and Sankha (Virata's eldest son), and finally used a divine weapon to slay Sveta. It was Sveta's death that signaled to the Pandavas that they had lost day 1, and depressed Yudhisthir at the end of the day, and Krishna had to boost their morale. Also, like they did later w/ Abhimanyu, Dushashan and other Kaurava warriors danced w/ glee at Sveta's death, they were that afraid of losing the war on day 1 itself on his account
Vrishasena
It's easy to overlook him, given how overflowing w/ military talent the Kaurava army was. Yet, he defeated every Pandava who fought against him except Arjun and Abhimanyu, and the day he entered the war, he defeated Draupadi's sons who launched a combined attack on him, and Abhimanyu had to salvage him
Iravana
Short lived, but he got introduced to his father on day 7 or day 8, killed a few of Shakuni's brothers pretty quickly, then got into a battle w/ the rakshasha Alambusha, and was getting the better of him when the latter resorted to mayavi tricks to kill him. Apparently, he was loved by Ghatotkacha, who was furious at his cousin's death
Shalya
Like Vrishasena, he was overshadowed by so many stalwarts in the Kaurava army, but he was no small potatoes. On day 1, he killed Uttar, and on several occasions, he preoccupied Bhima in battle, since his primary weapon was a mace. (In fact, in Draupadi's swayamvara, he was one of those who objected to Draupadi marrying a Brahmin (Arjun in disguise) and fought a disguised Bhima while Karna fought a disguised Arjun). On day 18, he was not killed in a single man to man combat w/ Yudhisthir: he was attacked by a combination of Bhima, Satyaki, Nakul, Sahadev, Dhrishtadyumna and Shikhandi. Each of them broke one weapon of his after another, finally his chariot was downed and Yudhisthir finally hurled a javelin at him that pinned him to the ground and slew him. What was tragic about him was him being tricked into joining the wrong side. Incidentally, it's wrong to accuse him of being a 5th columnist in the Kaurava army given his role as Karna's charioteer: he was only that on day 17, and the other 17 days, he fought wholeheartedly and lost his entire family and army in the effort.
These 6 stood out for me just now. I noted the Kurukshetra warriors: don't recall other underrated characters from other parts of the story. Subhadra did have an important role in not just raising Abhimanyu, but also in being the regent/caretaker of both Parikshit and Vajra when the Pandavas finally went on their swargarohan yatra. But I'm not sure that that qualifies as underrated