An odd thing to want to do -- if you were translating to another language and wanted to overwrite another character then that is one thing but to change to another normal character is odd. Was it like that in an older game or something?
I don't know the specifics of what goes here and there are many ways it could play out.
I am assuming it is just a way that the game differentiates between two of the same enemies in a given battle.
First thing I would look at is to find the rest of the battle text in the ROM and see what it does -- hopefully there is something more substantial than "?? * attacks!" and instead "?? * uses a potion" or something you can more easily use relative search for.
For a DS game I would expect some kind of placeholder in the text (could be binary, could be text based) and for it to be generated for the battle, or possibly even in real time. I would then use this data from the ROM to find where this text that does not change lands in memory during a battle.
Once you have that you have to figure out where the items put in there are located. Once you have that then you would want to find out how that is generated (probably want to note where it is in the memory and get into a new battle so new data can be generated and you can follow it through it all). Chances are the game will just add 1 or something to a value for each of the same enemy. Should not be too hard then to get it to add 20 instead and land on whatever character you need it to. Might even be able to add, or more likely force/set, that to the first value it uses and then the +1 stuff it might already do to generate the identifying character for the next identical enemy will be sorted without you having to do anything beyond that initial value. If you want something that is not in alphabetical/font order (P not being after R or S) then it gets a bit more fun, though the same initial steps apply and you just have to work more with the +1 stuff.
It should not change the game beyond the cosmetic but do make sure to test attacks, summons, magic, items and special abilities (and whatever else the game might allow you to target enemies with) as I have been surprised in the past.
Looking at the font there I doubt you will be lucky and find the game uses a specific font/encoding for that character you want to change -- very occasionally games will have an identifying character be a part of the font that is only used for identification and thus you could overwrite the font with whatever you like, however here it would probably leave you with garbled monster names.
Edit. Just saw the same thread over at romhacking.net was answered with the more technical stuff already done
http://www.romhacking.net/forum/index.php?topic=28661.0
