"Supporting Character". I would just leave it at that.

Terms like "False Protagonist" and "False Antagonist" cover such people when they switch sides midway. I think I stop bothering with labels once it gets beyond that level of complexity.

...
Regards,
Eric Fretheim
Assistant Prep Leader, 2015 October NaNoWriMo Prep Challenge

"It is perfectly okay to write garbage-- as long as you edit brilliantly." ~C.J. Cherryh

“No, writing 50,000 words in a month is normal. You are not crazy. This is not insane.” ~Teri Brown