No one is born knowing about any of this stuff, and it's easy to get a little confused when things are new. ? To make sure we set things up as you want them, could you please give us a little more info on the below:
What do you want to happen when someone goes to the papasenpai.heliohost.us website? If you want them to see the exact same thing as you have on senpai.wtf, because you are using the exact same single set of files for both web addresses, then this means you'd want an alias.
If you don't even want to use the web address papasenpai.heliohost.us at all anymore, and you just want people to visit senpai.wtf, then what you'll want us for us to change your main domain (papasenpai.heliohost.us) to be senpai.wtf. In this case, the papasenpai.heliohost.us address will no longer exist, and visitors will get a Site Not Found. You would only have a folder inside Plesk to put files for your senpai.wtf website.
Right now, you have papasenpai.heliohost.us as your main domain, and senpai.wtf as your addon domain. If you wanted to, you could leave things like this. (You can have up to 10 domains on an account, so having these 2 is no trouble.) The current setup is that both your sites need their own set of files. So this is good if you want to make 2 completely different sites, but not great if you want visitors to see the exact same thing, since your files would need to be in 2 places (and this is the problem that aliases will solve!)
Since you have a custom domain, I feel it's more likely you'd only want to share the senpai.wtf address for your site, so I suspect what you want is a main domain change, but please let us know. Some users (including myself) use the free heliohost domain as a testing site to try new things, and then also have a custom domain as an addon domain, where we keep the pages we want to show off on the web address we share with others, like the setup you have now.