My theory is Dean Harkin...why....well lets whittle it down

Dean
Definition: a man who is the SENIOR member of a group

Harkin
- The surname Harkins is an anglicised form of the patronymic Irish
surname "O hEarcain", which is derived from the word "earc" meaning
"red". Thus the surname would have originally been born by "a descendent of Red". First name origin & meaning:
Irish: Dark red
First name variations: Harkan, Harken, Harkon, Harkyn
Last name origins & meanings:
Irish:
Anglicized form of Gaelic Ó hEarcáin ‘descendant of Earcán’, a byname
or personal name formed from a diminutive of earc ‘red’, ‘bloody’

He is Har (S1 Ep11)- Harkin!!! Dean Harkin is Red John!!!

CASE CLOSED!!