Each group backs the ultimate character, one showing huge enthusiasm (6)
I believe the answer is:
zealot
'one showing huge enthusiasm' is the definition.
'each group backs the ultimate character' is the wordplay.
'each' becomes 'ea'.
'group' becomes 'lot' (lot is a kind of group**).
'backs' means one lot of letters go next to another.
'the ultimate character' becomes 'z' (letter at the end of the alphabet).
'ea'+'lot'='ealot'
'ealot' put after 'z' is 'ZEALOT'.
(Other definitions for zealot that I've seen before include "A fanatic or extremist" , "Over-zealous person, fanatic" , "Person full of enthusiasm, a fanatic" , "A fanatic or obsessed follower" , "Fervent, even militant, proponent of something" .)