The theme of otherness in Unamuno is one of the most interesting topics of his thought (for me it is so) and this is difficult to consider in a thought that had the ability to reason always by an alternate way and to see unprecedented senses in its reality.
Rimbaud's phrase has been leit motiv from my theoretical readings on representation and imagination, as well as the theme of realization through some other (or otherness, and not necessarily human). When I found myself working with a professor who was well versed in these matters, French translator Luis Miguel Isava, warned me about Rimbaud's phrase that a more accurate Spanish translation would be: Yo es otro, which would undoubtedly extend its meaning to other implications.
As you well point out, a long list of notable authors have internalized otherness in their works. However, for me, someone who explores the limits of this subject is Juan Carlos Onetti, in his novel La vida breve, and in general in all of his novels grouped under the label Ciclo de Santa María, where the otherness that Brausen becomes, through Díaz grey and Arce, his two vicars, one that he makes from a cinematographic script and the other in which he becomes, both as forms of certain perversions, they construct alterity as imaginary acts that reach their peak in the realization, in the plane of reality, of the town of Santa María, with its streets, traditions, stories, heroes, people and miseries.
Undoubtedly a great theme to explore from Unamuno and from a whole constellation of universal thinkers and artists.
Thank you, @josemalavem, for sharing this text and @adsactly for publishing it.