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Gwenno was one of the main characters you could have in your party
in the old, single player-Ultimas. Like Ultima V for example.
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No, not in your party, and not in Ultima V. As others pointed out, she
was Iolo's devoted wife, but she wasn't a member of your party. I can't
recall if she actually shows up as a character in V or VI. But in Ultima VII,
Part 1, The Black Gate, she is definitely mentioned as having gone off
elsewhere, though she is not a part of the game fiction itself (which
involved that evil Fellowship and the appearance of the Guardian). In
Ultima VII, Part 2, Gwenno is in the new land (not Britannia) you are
exploring, and one of the side quests involves tracking her down for Iolo
and releasing her from a block of ice. Gwenno also makes a cameo appearance
in Ultima IX, where she is found living in Iolo's hut and has a few words to say.
If I recall, Ultima IX also had a decorated gravesite dedicated to the actual
woman who inspired the Gwenno character.