I'm not sure if I was aware of the two different text adventure versions at the time I originally wrote the first post here. Looking over the materials available to me now, I have to agree with what you've said. Eusis is much less common than I originally thought. It was just the first to become known in the west, thanks to Naflign getting the Game no Kanzume discs, and popularized by the Compendium translation.
From a quick glance at all the official materials available to me at the moment, these are all the different ways I can find Rolf's name presented:
- Original Text Adventure - Yushis on title screen, Yousis in the menu
- Game no Kanzume port of Text Adventure - Eusis
- Collection book (pictured above) - Yousis
- Complete Collection - Yusis (in image filenames), plus the original Text Adventure
Everything else uses ユーシス. One book actually lists "Rolf" alongside that. The game manual seems to use 主人公, if I read it correctly. Google translates that as "the hero."
Now, I'm inclined to consider Yusis, from the Complete Collection image filenames, to possibly be less valid. It tends to use some very usual names on some characters. Examples include Ludy for Chaz, Slay for Rune, and Hyui for Hugh. The less said about it using Kain for Rhys' name, the better.
So, I guess Yousis is actually the most used form of the name after all. There are so many different forms, though...