As most of you probably know, certain recruitable enemies in FE6 don't receive hard mode bonuses on hard mode (Raigh, Hugh, and Douglas). Originally, none of them were supposed to receive those bonuses (or so I heard), but units loaded as reinforcements skip the following check, and therefore get them.
At 666C78, there are 8-byte entries consisting of a word-length character id and a word-length number of extra levels to gain (which, in vanilla, is always 0). The table is terminated with a 00 character id. The entries are as follows:
0x17 - Rutger
0x35 - Cath
0x16 - Fir
0x04 - Shin
0x38 - Gonzalez
0x0F - Klein
0x2E - Tate
0x2A - Raigh
0x37 - Miledy
0x3F - Cecilia (don't ask me why, I have no idea)
0x1D - Perceval
0x1A - Garret
0x27 - Hugh
0x32 - Zeiss
0x0B - Douglas
If the character isn't the in the table, then the game gets the usual hard mode byte (byte 0x13 in chapter data).
REMINDER: The following only works for enemy characters that are loaded in the initial main block. Removing Cecilia from this list won't do anything because she doesn't start out as an enemy.
If you want to make a character get a certain number of extra levels in hard mode, change byte 4 to the number of levels you want. If you feel Douglas merits 10 levels, then his entry would look like
0B 00 00 00 0A 00 00 00
If you want to make a character get the same levels as the rest of the people in the chapter, change their character byte to someone you know won't get any (don't make it 00, because then all entries after won't get read).
Hugh with hard mode bonuses (in chapter 16, 11):
For those who may be wondering 'where did all that mag come from? he only has a 30% mag growth', remember that class growths are used here, not character growths. The male mage has a respectable 55% magic growth, meaning with 55x11=605 (+/- some number that I forget), he's guaranteed to get at least 5 and probably 6 mag.