Not a single word, but re your "phrase-requests" tag, the phrase/cliché
“through thick and thin” means:
through good times and bad times
and is used today primarily to convey
the idea of supporting something or someone in all circumstances.
(from The Free Dictionary by Farlex, citing ‘McGraw-Hill Dictionary of American Idioms and Phrasal Verbs’ and ‘The American Heritage® Dictionary of Idioms by Christine Ammer,’ respectively).
In your example it could be used as follows:
“I am/will remain his/her fan through thick and thin.”