Wikipedia states that Aloysius is:
... a Latinisation of the names Louis, Lewis, Luis, Luigi, Ludwig, and other cognate names (traditionally in Medieval Latin as Ludovicus or Chlodovechus), ultimately from Frankish *Hlūdawīg, from Proto-Germanic *Hlūdawīgą ("famous battle").
Looking at the list of Latinised names you can see a lot of Latinised names ending in -is, -us, and -ius. This kind of make sense, as these seem to be pretty common declensions in Latin.
However, I am puzzled with the "y" in between Aloysius. From that list (perhaps excepting the conversion from English names, where an "y" in the original name is more common), there are very few exceptions having the "y" in the Latinised name. For example, Syncerus (Sannazaro).
If there is one, what was the logic of the Latinisation of the name Louis as Aloysius? I mean, given my current little knowledge of Latin, works with "y" and "k" tend to be mainly of Greek origin. So it seems rather unatural to translate a name with a "y", even more if the original did not have it. What was wrong with Aloisius? It seems "oi" is actually a diphthong, whereas "oy" is not.