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The superlative of selfish is "most selfish" and the comparative is "more selfish".
most selfish, more selfish
The superlative degree of selfish is "most selfish", and the comparative degree is "more selfish".
If she wants to be selfish,you can be selfish too.
One possible prefix for "selfish" is "un-", which would create the word "unselfish," meaning not selfish.