U-21 is an unicorn stallion, and was a breeder in Stable-99. A fellow obsolete breeder like P-21 and escapee from Stable 99, U-21 joins the raiders attacking the Stable and becomes a recurring antagonist, albeit a fairly inept one.
Seeing a path P-21 easily could have followed in him, Blackjack is repeatedly unable to kill U-21 despite ample opportunity. He continually to tries to prove his worth to the forces seeking E-1101 and Blackjack, joining in failed attempts to capture her pip-buck and take revenge on one of his tormentors from the Stable.
He was killed by Gem and Mini in an ambush after finding Blackjack with P-21's help.