My neighbor has been “warning” me that my Serperior (who I raised from a Snivy) is potential seconds away from constricting me, lies next to me to measure if she can eat me, etc. No attempt to talk some sense into him has helped. I don’t personally mind, but I worry that he’s stressing out my Serperior, or worse, that he might try to “save” me from her himself. Are either valid concerns? I don’t want to have to keep her in her ball around my own home for her safety if I can help it.

Besides the unlikelihood of a pokémon eating the trainer that has raised it from birth, serperior aren’t major predators to begin with. They photosynthesize and supplement their diets with far smaller prey (insects mostly). Prey as large as a human would actually likely kill a serperior because they don’t have the digestive system for it.

However, your concern about your neighbor stressing out your serperior is actually valid, as if he’s constantly measuring her and so forth, they may be violating her personal space, which could actually make her skittish enough to attack. Luckily, though, this issue can be solved with a restraining order (unless you actually like said neighbor).