In a story I'm working on one character has a pet name for another. The full name is Marisa, while the pet name is Risa. I'm wondering if the first time "Risa" is used whether I should have a leading apostrophe so that it is very clear that the intermixing is deliberate and not some screw-up like I changed the name partway through and didn't find all uses. I might use the leading apostrophe in all cases but I am blind and don't like how my speech synthesizer treats that so would really rather not.
If it matters to the answer there is a huge power differential between the pair, the one using the pet name is a captured princess reduced to concubine while the other is a literal slave. The princess would be a slave too except that the victorious captor can't afford that for political reasons.