Roma is a fictional character featured in numerous Marvel Comics titles. She is the daughter of Merlyn. Roma is the Omniversal Guardian, charged with the safety of the Omniverse. She is assisted in her task by Saturnyne, the Omniversal Majestrix and the Captain Britain Corps.
Roma and her father exist in another dimension called Otherworld. Captain Britain, or more specifically, his spirit, met them in this dimension after a battle with a villain named Lord Hawk that left him in a coma.
Aside from bequeathing Captain Britain with great physical strength, Roma also originally gifted him with a quarterstaff, a weapon that could be used for direct attack or emit a protective force field. Merlyn eventually replaced the staff with Captain Britain's Star Scepter.
Roma would eventually replace her father as the Omniversal Guardian and ruler of Otherworld, thinking her father had died. In fact, Merlyn had faked his death so he could manipulate future events while being safe from his enemies.
During this encounter, Roma was really being held captive in her Starlight Citadel by the Trickster (aka the Adversary), a deity who planned on destroying the earth to remake it into whatever image he chose. The X-Men defeated the Adversary, freeing Roma and forcing the Adversary into a portal. Using Forge's spell, it took harnessing the souls of all eight active X-Men and Madelyne Pryor, who was there at the time, to create and shut the portal. Roma repaid them by retrieving their souls from the portal, which had a side effect of making them undetectable to electronic equipment and cameras, effectively making them "invisible" to their enemies.
Roma also bequeathed the X-Men with a portal called the Siege Perilous. This portal is a gem that enlarges into a doorway. Individuals walk through the portal to have their lives and deeds judged on a cosmic scale, eventually being reborn if they are deemed worthy. X-Men who have traveled through the Siege Perilous include Havok, Rogue (accidentally sucked into it while battling the Master Mold/Nimrod in issues 246-247), Psylocke, Dazzler, and Colossus.
Merlyn in disguise, manipulated Captain Britain into confronting Roma and she removed the jinx and told him about the secret threat. Excalibur encountered the threat shortly afterwards: Necrom, Merlyn's teacher had been planning for millennia to use the Phoenix Force to increase his own power. The combined powers of Excalibur defeated Necrom as Merlyn had planned. Roma told Excalibur that from now on, they would be free to determine their own destiny.
Roma was captured and impersonated by the villain Mastermind, who planned on using the Sword of Might and the Amulet of Right to reshape the Omniverse. Mastermind was defeated by Captain Britain and his friends and Roma gave the throne of Otherworld to Captain Britain, who now wielded both the Sword and the Amulet. Roma would retain her function as Omniversal Guardian though.
Roma briefly appeared in Fantastic Four when she sent Gatecrasher to kidnap Franklin Richards, thinking he was too powerful to stay on Earth and with his family. Johnny Storm was later able to convince her otherwise.
She again appeared when Abraxas was trying to find the Ultimate Nullifier. She summoned Valeria Richards and Franklin Richards to protect them from Abraxas but was unsuccessful, since she was murdered in the process. But Franklin revealed that Roma had taught him for an unknown period of time and that Valeria and himself would be needed in resurrecting Galactus. They used up all their powers to resurrect Galactus and ended the threat of Abraxas. Roma, who was killed, was ultimately resurrected in the process.
Roma is the youngest member of her race, but still millennia old. She is immortal and has vast magical powers at her disposal, which she can use in countless ways, including interdimensional teleportation, astral projection, healing, mind-reading, energy manipulation and many more. As the Omniversal Guardian she also has access to the technology and power of Otherworld, which has devices capable of destroying complete universes. The Captain Britain Corps obeys her commands.