An unnamed damsel whom Phyllis Ann Karr calls Malvis, and I follow her example.
This damsel arrived at Arthur's court "on message from the great lady Lile of Avelion", girded with a cumbersome, uncomfortable sword. She said she was looking for a knight who could draw the sword from its scabbard and thus relieve her of it,
but he must be a passing good man of his hands and of his deeds ... a clean knight without villainy, and of a gentle strain of father side and mother side.
Certes, said the damosel, this is a passing good knight, and the best that ever I found, and most of worship without treason, treachery, or villainy, and many marvels shall he do. Now gentle and courteous knight, give me the sword again. Nay, said Balin. ... Well, said the damosel, ye are not wise to keep the sword from me, for ye shall slay with the sword the best friend that ye have, and the man that ye most love in the world, and the sword shall be your destruction. I shall take the adventure, said Balin ... but the sword ye shall not have ...
Ye shall repent it within short time, said the damosel, for I would have the sword more for your avail than for mine, for I am passing heavy for your sake; for ye will not believe that sword shall be your destruction, and that is great pity. With that the damosel departed, making great sorrow.
I take no force though I have both their heads, for he slew my brother, a good knight and a true, and that gentlewoman was causer of my father's death.
Arthur refused "Nineve's" request, but Balin, learning of it, came and lopped off "Nineve's" head, to the King's chagrin. Balin explained that he had been seeking her for three years for having caused the death of his mother, that
this same lady [of the Lake] was the untruest lady living, and by enchantment and sorcery she hath been the destroyer of many good knights.
After Balin's departure, Merlin arrived. Being told all that had happened, the Mage explained:
[T]his same damosel that here standeth [did stand?], that brought the sword unto your court, I shall tell you the cause of her coming: she was the falsest damosel that liveth. Say not so, said they. She hath a brother, [said Merlin] a passing good knight of prowess and a full true man; and this damosel loved another knight that held her to paramour, and this good knight her brother met with the knight that held her to paramour, and slew him by force of his hands. When this false damosel understood this, she went to the Lady Lile of Avelion, and besought her to help, to be avenged on her own brother. And so this Lady Lile of Avelion took her this sword that she brought with her, and told there should no man pull it out of the sheath but if he be one of the best knights of this realm, and he should be hard and full of prowess, and with that sword he should slay her brother. ... Would God she had not come into this court, but she came never in fellowship of worship to do good, but always great harm.
If, in this welter of accusation and counter-accusation, we take Merlin's tale as sooth, then "Malvis" was a wicked woman. Personally, I hold Merlin's testimony suspect. By Balin's own account, which seems reliable - Merlin himself calls Balin a good knight - and by "Nineve's" own vengeful request, the dead Lady of the Lake had been wicked. But "Nineve" appears to have been a friend of Merlin himself, and a long tale of the other damsel's wickedness may have been Merlin's sagest way of taking attention off "Nineve's" wickedness and his own possible guilt by association with her. Moreover, when we look for the fulfillment of Merlin's prophecy that Balin would slay the damsel's brother with the sword, we find as about the only named candidates for her brother to be Sir Garlon and Sir Balan.
It is possible that "Malvis" was indeed the sister of both Balin and Balan; the fact that they did not recognize each other as siblings does not negate the chance, for the romances are full of such cases - one sibling leaves home while the other is still an infant, and so on (compare with Lancelot and Ector de Maris, Gawaine and Gareth). It is also possible that the damsel's brother was one of the knights killed in battle against Ryons, Nero, and Lot, or that Malory omitted the death in question completely.
It is even conceivable that Sir Lanceor was "Malvis" brother, although Balin slew him with lance rather than sword. Garlon, however, seems the likeliest knight for the brother of Merlin's tale, and Garlon's patent villainy hardly tallies with Merlin's praise of the damsel's brother. Moreover, although the damsel's praise of Balin may have been a ruse to tickle his vanity and ensure that he would keep the sword, "Malvis" subsequent efforts to get it back from him and prevent the coming tragedy sound sincere to me. Since I can find nothing more about the damsel, unless she is to be identified with one of Malory's other ladies, the question of her guilt or innocence may perhaps be considered open.
Balin's Sword | The Legend of King Arthur