Most alive in reduplication. 忠心耿耿 means staunchly loyal, while 耿耿于怀 means to brood over something. 耿直 describes a frank, blunt personality. As a surname it is read the same, 耿.
Left ear radical. The original sense pictured an ear against the cheek; that physical image faded, leaving the radical as the indexing component while the meaning shifted to 'bright' and then 'upright'.
Right 火 gives the 'bright, shining' core idea - light from fire. From visible brightness the meaning extended to a clear, transparent character, hence 'honest' and 'upright'.
