Hi there! Unless you are copy-pasting any text written about this character without crediting the original writer, this is not considered plagiarism. You are definitely allowed to base a character off of an existing character, especially if you are then putting your own unique spin on it. If you want to be safe, you can state somewhere in your SU "my character is loosely based on ________." It's up to the individual GM whether or not they'll allow you to base your character on a pre-existing one, though!
I hope that answers your question! :>