There's no easy way to do so in game that I'm aware of, including debug mode. But Era games are hand-editable and don't require recompilation, so if you want to do this badly enough, you can. I haven't learned how to do this myself, but I can tell you the general steps that are probably involved.
If you want to give a character with no dialogue dialogue, then you need to find out how the characters are pointed to specific files in ERB/口上. You'll have to find the specific character you want in that list*, edit the code referencing which ERB/口上 file to call for their dialogue (based on the rest of the code, it's probably something like a CASE NO:XX branch, with everyone else relegated to a CASEELSE that calls the generic dialogue file), and then point it to a particular dialogue file. This can be an existing one or a new one. This is hard mode, but probably isn't actually that complicated. A lot of Find in Files sleuthing will be involved. (If you don't have access to Find in Files search, download Notepad++.)
* (with the debug mode command "no:XX" where XX is their number in your roster, so no:0 gives you the internal value assigned to your main character in the character database, and no:XX gets the first character recruited, which will either be your starting slave selection or a character carried over in NG+. Custom portraits are a good intro to this idea, so look in the folder resources/custom portraits for info)
Easy mode is altering the dialogue of someone who already has dialogue. Pick out 2-3 of their training lines and Find in Files through ERB/口上 for the right file, then alter it appropriately. If you're feeling fancy, use IF TALENT:XX (where XX is the Japanese name of a talent, like Timid or Docile or High Pride) to branch. Dialogue sets are frequently shared, so if you decide to alter Remiel's dialogue with the translated Lady set in EVENT_PUB306_女_淑女_ALL.ERB, for example, then both Julia and Byakuren will also have the changed dialogue, because they also use that set. But if you set it so that only a character with three specific talents gets one set of dialogue, then you'll have fewer unintended dialogue changes. You can use the Japanese wiki at
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to find the names of those talents.
Being able to do some work on an Era game yourself is useful when, for example, you find that "&" in an enemy name ("Tsukiyomi&Susanoo") makes the game crash. Unless there's a new EraMegaten release in the last couple of days, that bug's still in the current one. (It's in Hikari's ending of the Revue Starlight crossover quest.)