It should be mentioned that the question is about a serial. Even if 'Friend' was in quotes, we could understand. Then 'who' is also misleading. If it is about serial, 'which' should be used. If it is about friends in general. 'what' should be used.
So, if it is about serial, I request, please edit and write- Which is the most unforgettable character in 'Friends' serial
G. K. Ajmani Tax consultant http://gkajmani-mystraythoughts.blogspot.com/